Вставка текста в PyQt приложение

Я не знаю зачем вам строка

self.label_4.resize(self.window_4.label_4.sizeHint())

проблема в ней.

P.S. Чтобы получать реальную ошибку всегда запускайте свое приложение в CMD/терминале/консоли.

import sys

from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtWidgets import QApplication, QMainWindow


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(816, 460)
        MainWindow.setMinimumSize(QtCore.QSize(816, 460))
        MainWindow.setMaximumSize(QtCore.QSize(816, 460))
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.gridLayoutWidget = QtWidgets.QWidget(self.centralwidget)
        self.gridLayoutWidget.setGeometry(
            QtCore.QRect(10, 10, 801, 441))
        self.gridLayoutWidget.setObjectName("gridLayoutWidget")
        self.gridLayout = QtWidgets.QGridLayout(self.gridLayoutWidget)
        self.gridLayout.setContentsMargins(0, 0, 0, 0)
        self.gridLayout.setObjectName("gridLayout")
        self.horizontalLayout = QtWidgets.QHBoxLayout()
        self.horizontalLayout.setObjectName("horizontalLayout")
        self.verticalLayout = QtWidgets.QVBoxLayout()
        self.verticalLayout.setObjectName("verticalLayout")
        self.task_4 = QtWidgets.QPushButton(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(18)
        self.task_4.setFont(font)
        self.task_4.setObjectName("task_4")
        self.verticalLayout.addWidget(self.task_4)
        self.task_7 = QtWidgets.QPushButton(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(18)
        self.task_7.setFont(font)
        self.task_7.setObjectName("task_7")
        self.verticalLayout.addWidget(self.task_7)
        self.task_9 = QtWidgets.QPushButton(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(18)
        self.task_9.setFont(font)
        self.task_9.setObjectName("task_9")
        self.verticalLayout.addWidget(self.task_9)
        self.horizontalLayout.addLayout(self.verticalLayout)
        self.verticalLayout_2 = QtWidgets.QVBoxLayout()
        self.verticalLayout_2.setObjectName("verticalLayout_2")
        self.task_10 = QtWidgets.QPushButton(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(18)
        self.task_10.setFont(font)
        self.task_10.setObjectName("task_10")
        self.verticalLayout_2.addWidget(self.task_10)
        self.task_11 = QtWidgets.QPushButton(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(18)
        self.task_11.setFont(font)
        self.task_11.setObjectName("task_11")
        self.verticalLayout_2.addWidget(self.task_11)
        self.task_12 = QtWidgets.QPushButton(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(18)
        self.task_12.setFont(font)
        self.task_12.setObjectName("task_12")
        self.verticalLayout_2.addWidget(self.task_12)
        self.horizontalLayout.addLayout(self.verticalLayout_2)
        self.verticalLayout_3 = QtWidgets.QVBoxLayout()
        self.verticalLayout_3.setObjectName("verticalLayout_3")
        self.task_13 = QtWidgets.QPushButton(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(18)
        self.task_13.setFont(font)
        self.task_13.setObjectName("task_13")
        self.verticalLayout_3.addWidget(self.task_13)
        self.task_14 = QtWidgets.QPushButton(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(18)
        self.task_14.setFont(font)
        self.task_14.setObjectName("task_14")
        self.verticalLayout_3.addWidget(self.task_14)
        self.task_15 = QtWidgets.QPushButton(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(18)
        self.task_15.setFont(font)
        self.task_15.setObjectName("task_15")
        self.verticalLayout_3.addWidget(self.task_15)
        self.horizontalLayout.addLayout(self.verticalLayout_3)
        self.gridLayout.addLayout(self.horizontalLayout, 2, 0, 1, 1)
        self.title_label = QtWidgets.QLabel(self.gridLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(26)
        self.title_label.setFont(font)
        self.title_label.setObjectName("title_label")
        self.gridLayout.addWidget(self.title_label, 1, 0, 1, 1)
        MainWindow.setCentralWidget(self.centralwidget)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(
            _translate("MainWindow", "MainWindow"))
        self.task_4.setText(_translate("MainWindow", "Задание №4"))
        self.task_7.setText(_translate("MainWindow", "Задание №7"))
        self.task_9.setText(_translate("MainWindow", "Задание №9"))
        self.task_10.setText(_translate("MainWindow", "Задание №10"))
        self.task_11.setText(_translate("MainWindow", "Задание №11"))
        self.task_12.setText(_translate("MainWindow", "Задание №12"))
        self.task_13.setText(_translate("MainWindow", "Задание №13"))
        self.task_14.setText(_translate("MainWindow", "Задание №14"))
        self.task_15.setText(_translate("MainWindow", "Задание №15"))
        self.title_label.setText(_translate("MainWindow",
                                            "Тренажёр для подготовки к ЕГЭ по русскому языку"))


class Ui_MainWindow4(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 445)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.label_4 = QtWidgets.QLabel(self.centralwidget)
        self.label_4.setGeometry(QtCore.QRect(20, 10, 696, 95))
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(20)
        self.label_4.setFont(font)
        self.label_4.setObjectName("label_4")
        self.outlabel_4 = QtWidgets.QLabel(self.centralwidget)
        self.outlabel_4.setGeometry(QtCore.QRect(20, 340, 91, 31))
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(20)
        self.outlabel_4.setFont(font)
        self.outlabel_4.setObjectName("outlabel_4")
        self.n_btn_4 = QtWidgets.QPushButton(self.centralwidget)
        self.n_btn_4.setGeometry(QtCore.QRect(560, 390, 198, 39))
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(20)
        self.n_btn_4.setFont(font)
        self.n_btn_4.setObjectName("n_btn_4")
        self.verticalLayoutWidget = QtWidgets.QWidget(
            self.centralwidget)
        self.verticalLayoutWidget.setGeometry(
            QtCore.QRect(20, 120, 191, 201))
        self.verticalLayoutWidget.setObjectName(
            "verticalLayoutWidget")
        self.verticalLayout = QtWidgets.QVBoxLayout(
            self.verticalLayoutWidget)
        self.verticalLayout.setContentsMargins(0, 0, 0, 0)
        self.verticalLayout.setObjectName("verticalLayout")
        self.label_41 = QtWidgets.QLabel(self.verticalLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(20)
        self.label_41.setFont(font)
        self.label_41.setObjectName("label_41")
        self.verticalLayout.addWidget(self.label_41)
        self.label_42 = QtWidgets.QLabel(self.verticalLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(20)
        self.label_42.setFont(font)
        self.label_42.setObjectName("label_42")
        self.verticalLayout.addWidget(self.label_42)
        self.label_43 = QtWidgets.QLabel(self.verticalLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(20)
        self.label_43.setFont(font)
        self.label_43.setObjectName("label_43")
        self.verticalLayout.addWidget(self.label_43)
        self.label_44 = QtWidgets.QLabel(self.verticalLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(20)
        self.label_44.setFont(font)
        self.label_44.setObjectName("label_44")
        self.verticalLayout.addWidget(self.label_44)
        self.label_45 = QtWidgets.QLabel(self.verticalLayoutWidget)
        font = QtGui.QFont()
        font.setFamily("Times New Roman")
        font.setPointSize(20)
        self.label_45.setFont(font)
        self.label_45.setObjectName("label_45")
        self.verticalLayout.addWidget(self.label_45)
        self.lineEdit = QtWidgets.QLineEdit(self.centralwidget)
        self.lineEdit.setGeometry(QtCore.QRect(110, 340, 651, 31))
        self.lineEdit.setObjectName("lineEdit")
        MainWindow.setCentralWidget(self.centralwidget)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(
            _translate("MainWindow", "MainWindow"))
        self.label_4.setText(_translate("MainWindow",
                                        ""))
        self.outlabel_4.setText(_translate("MainWindow", "Ответ:"))
        self.n_btn_4.setText(
            _translate("MainWindow", "Отправить ответ"))
        self.label_41.setText(_translate("MainWindow", "1 слово"))
        self.label_42.setText(_translate("MainWindow", "2 слово"))
        self.label_43.setText(_translate("MainWindow", "3 слово"))
        self.label_44.setText(_translate("MainWindow", "4 слово"))
        self.label_45.setText(_translate("MainWindow", "5 слово"))


class ForthTask(QMainWindow, Ui_MainWindow4):
    def __init__(self):
        super().__init__()
        self.setupUi(self)
        self.setWindowTitle('Задание №4')
        self.st_t4()
        self.n_btn_4.clicked.connect(self.st_t4)

    def st_t4(self):
        self.label_4.setText(
            'В одном из приведённых ниже слов допущена ошибка в \nпост'
            'ановке ударения: НЕВЕРНО выделена буква, \nобозначающая '
            'ударный гласный звук. Выпишите это слово.')

# уберите строку ниже            
#        self.label_4.resize(self.window_4.label_4.sizeHint())
# или так
        self.label_4.resize(self.label_4.sizeHint())


class Application(QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()

        self.setupUi(self)
        self.setWindowTitle(
            'Тренажёр для подготовки к ЕГЭ по русскому языку')

        self.task_4.clicked.connect(self.forth_task)

    def forth_task(self):
        self.window_4 = ForthTask()
        self.window_4.show()


if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = Application()
    ex.show()
    sys.exit(app.exec())