Good question I wonder since the take down controls are totally different. Paclite says their uppers fit 1-3 and 22/45 series. They also offer an accessories page for MK IV and no mention of their uppers working with IV so I'd assume no?
Just called Ruger. MK III upper will not work with MKIV lower. Some internals are interchangeable, and magazines from the steel framed MK series are interchangeable but not 22/45 mags. From the horses mouth.