как привести 2 последних символа toUpperCase и 2 первых toLowerCase в строке в Java?

StringBuilder builder = new StringBuilder("HEllo world");
builder.setCharAt(0,
        Character.toLowerCase(builder.charAt(0)));
builder.setCharAt(1,
        Character.toLowerCase(builder.charAt(1)));
builder.setCharAt(builder.length() - 1,
        Character.toUpperCase(builder.charAt(builder.length() - 1)));
builder.setCharAt(builder.length() - 2,
        Character.toUpperCase(builder.charAt(builder.length() - 2)));
String result = builder.toString();