This is just my personal opinion, but I think the measly extra $10 for the 8GB will benefit you in the long run.
We have no idea what games will require on-board memory for DLC (some will go on the game card), and you never know how big some system updates may be. Also, some demos I've seen are 1-2GB.
That's half your card, not including any saves. 4GB IMO is almost unusable unless you're strictly never going to connect your Vita to wifi whatsoever.
99.9% of phones that can tether have GPS capabilities. They also have multiple GPS options, of which the Vita only has one.