'Forward Compatibility' lads, seems to be the in thing too as that's what MS were saying when they mentioned it for the XO a couple of weeks back - you don't need to buy it as all games will still run on regular PS4.. just if you want 4K.... It also mentioned (from the Sony camp) that it'll help with PS VR, while MS are talking about making the XO similar to a PC, so not only games with added resolution, but perhaps more sampling, maybe even higher resolution textures, etc...
So not really the 32X fiasco all over again - probably more like the N3DS, or the N64 memory expansion (ahhh, memories!)
Doesn't mean to say I think it's a good idea though! Think we need a bit more information first - interesting stuff anyway, there seems to be a big margin for error with it if past addons are anything to go by!
One thing's for sure, and I said it when they were both released: PS4 and XO should've been more powerful in the first place! When you could buy a tower rig that was as powerful as the PS4, on its launch day?!? You had to shell out at least 3x the price for a similar specced tower as the Xbox 360 when that released.... pity they've only just figured this out...