My personal opinion is that the 8G is the way to go since it's only $10 more and you're doubling your space. Games are usually under 2G with some exceptions like Uncharted so you can easily fit 2-4 games on there and still have room for your game saves.
As for the 4G card... well... Uncharted is like 3.5G. Probably not going to be carrying a lot of digital games with one, that's for sure.
4 gigs will be enough to save a lot of games. Uncharted golden abyss uses 64 megs and I'm guessing that will be on the high end for most vita games. So as long as you're only saving files from a physical copy and not downloading full games, you should be able to save around 40-60 games.
8GB is the sweet spot right now, until SONY ends up bundling one in with the Vita, which I think is likely in the current financial climate. Maybe a good idea to wait for an 'Uncharted Bundle', can you tell I got burned by the 3DS ? Ha ha ha !
4gb is fine if you plan on saving your games, and 4gb/30mb=1333 game saves. More games than PS3 also feature music playback during your games. I know Little Deviants does not, but Dungeon Hunter Alliance and Uncharted do.
Don't know how Bobby came up with that math. With 4GB at 30mb per game that's 137 saves. That is pretty consistent with what I have found with my games. There are going to be a few that are higher obviously. With less than 100 games total out right now 4GB will be plenty for someone who only wishes to do game saves. However if you are goign to download any game I strongly encourage you to get a larger card. Especially since Sony recently released an update that they will be adding the promised PS One Classics support this summer.