When you say downsizing, you just mean space, right? How much would moving from 4x5 to 120 save you in terms of size, if you're not going to print? The space it takes to develop 4x5 isn't so much bigger than that for 120, and a 4x5 field cam ain't so much bigger than a Hassie. Why don't you keep at least one of the 4x5, and a few of your best lenses for it?
Having moved from DSLR to MF to 4x5 (and now dabbling with 8x10), I think you'll really miss the size.
Not to take away from Hasselblad. I love my 203FE and 903SWC Sell one 4x5 and 5x7 and some lenses, and pick up a 501CM + 50/80/150 fo cheap, and you'll have best of both worlds, IMHO :-)