The MKIV frame has either aluminum or stainless steel.
The MKIV 22/45 is a plastic frame.
I don't have a MKIV but I do have several MKII's.
I prefer the aluminum frame. They are lighter than the Stainless guns and I don't like plastic.
I have never liked the 22/45 pistols.
I agree the metal frames look 10X nicer. I have a 22/45 MKIV Tactical and while it may be ugly, its an excellent training gun. The regular MK series would not work as well for me since the grip angle differs from all my serious use guns.