Indeed T4 was actually inspired as the seperate budget terminal
aka
KUL with seperate LCCT ( for AirAsia and such) but now fortunately all part of KLIA2

Yet nowadays full service CX also operates from Changi's T4 which seems odd as they are seperated from the rest of OneWorld in T1
Which gives notion that T4 is moving from pure LCC to also handle full service legacies