Upconverted image from 1080p to 4k? Not sure if anyone remembers when all the 1080i/480p so called progressive scan projection TV's came out years ago, but EERYTHING that was upconverted looked "soft". I seam to remember similar results on our 720p HD flatscreen tvs, upconverting standard def signals off cable and Satalite!..SOFT LOOKING!
Would this be the case with 1080p HD material, scaled up to 4k? Well from what I briefly read about the 4K sony projector on upconverting blurays (It makes little to no sense why you'd need a 4K bluray upconverting player, when your 4K display/projector would already do this), apparently, it does a superb job! With lesser material, probably not as good.
Anyway, since 1080p already is very good, I'd presume scaling up would be just fine across the board. Probably more of an issue is whether spending big bucks on a 4K display makes sense, when there's no real native content to use, and the cost of these units are PRICEY at this point!
Im sure, by the end of next couple of years, this won't matter, and all TV's will be either 1080p, ULtra HD, or 4k, and such, and prices will be down to todays reasonable 720/1080 points. Then, all of this won't matter.
Until then, I don't think they could offer enough incentive for me, or the consumer market, to upgrade their units to 4k. Most of us are plenty happy with 1080p at reasonable viewing distances.