I know Pentax has had the 645 'cropped sensor' 4x5 out for quite a while - I watched an interesting youtube video comparing it with the d800e when I was trying to decide if I should buy the Nikon; I guess I discounted it because I haven't heard very many good things about it. I was really hoping for something that produced an image that was worth all the additional effort (weight, expense) to capture it. As for the lenses, my understanding is that much of the older 4x5 glass from decades of film cameras will still work (at least in full manual) - maybe to augment just one new crazy-expensive leaf-shutter lens.
I've read that it's possible to get good results in medium format by purchasing 30 yo film gear, processing the film then scanning it, for $500-$1000(?) - but that sounds like a LOT more effort than I'd see myself undertaking often enough to be 'in the right place at the right time' to really get outstanding shots on a regular basis, and over my pain threshold for a novelty.