4GB is the max cart size currently, so there won't be any game surpassing that anytime soon. it might change in the future though.
Developer can choose to make their game saved on the game card or to mem card. if it's saved to game card, then then you can't backup the save file. so it's probably saver to buy those games digitally.
one thing to note about Vita save data. they're not separate from the game data itself. so you can't just copy your save data and save it to your pc. you have to backup the whole game data.
basically, just for instance, you download Uncharted digitally and it have 3GB file size. when you play the game and save, instead of making a separate save file like previous PS device, it will instead add around 4mb to your game data, so now your game data is 3,004mb
so basically all save data is now tied to an account, I don't think you can copy friend save data to play on your game either, even if you own the game.