Darin...as you note. the shutter/lens unit on all three and likely IV are the same...single unit, helical movement of complete unit. But, you are right, 4.5 is fine, why I never switched the lenses. Both are same...Hexar/Seikosha units...cannot beat them. I get as nice prints with this wee rig as I did with Certo, Bessa, and my former great 67 Ikonta Mess, which I loved dearly but got enticed by the wee Pearl...so much smaller.
Only reason I have the extra unit is coincidence. I bought the camera and the lens unit came up at same time, What the hey, I thought for $50...you never know...