Ok I found that 4 GB in a K750 or a W800 does actually work (at least in a K750, that's for sure) one of my friends has one and it is loaded with an estimated 73%, and it works fine, but he has R1CA021 firmware, Generic, and works with patches, with gfx, with everything, you can buy one 4 yourself, but, like someone said, why 4GB? It's very much and useless sometimes, even my card of 2GB (I also have K750) is filled with a 30% of data, and I don't know what to put in it, but maybe in time, I will fully load it.

Have a nice day...

LE: oh, and I use Sandisk, I don't like the Sony's, but it might work better with a Sony...