There is no reason why clothes for V4 require Poser Pro.

But indeed there were MAT pose files for PP - for clothes and skin, but that was meant for Pro Pack, which some people call falsely Poser Pro.

Tthe Pro Pack (released in 2000) was an add on for Poser 4.

Before that *.bum files existed that are today *.jpg files, those who work as bump maps.

So it means if you use a very old Poser version that requires *.bum files (before Pro Pack) you cannot use today's clothes for V4.