If that combo is a must, just buy the current version and have the grip frame milled for removeable grips. There is at least one gunsmith that specializes in this but I'm sure any competent gunsmith can do it. Some folks do this themselves but that's beyond my skill set.
As an alternative, buy a 4" and a model with removeable grips. Swap uppers and lowers, then sell the one you don't want.
Have you tried an original 22/45 grip frame? It might meet your needs and it came in a 4" as well. You just have to look around a little to find one.
Highstandardguy, thanks for suggestions. I grew up when High Standards were popular and did they ever make nice pistols. Wish they were still around. In the end the 4" fixed grip 22/45 will be fine. I was just looking for a project gun and liked the idea of maximum flexibility. Had not thought of having it milled out My m,ain desire is trigger improvement and they works either way, at my age probably need an electronic sight. I still work full time and wanted a diversion even though I love my work. Ruger seems to have more aftermarket stuff.
I thought it would be fun to assemble just what I wanted. I had not thought about the top end swap, a possibility.