lp231 :
The amount of vram does not equal to performance. Vram is basically used for games that loads lots of textures.
GTX 760 4GB cost around $300, for that you can get a GTX 770 at 2GB or R9-280x 3GB
Depending on which GTX 770 4GB your getting, cheapest is around $360 while some are near $400 mark. For $400 the R9-290 will be the better option. It has 4GB of vram and it's a lot faster than the GTX 770
GTX 780 only comes in 3GB and there is no 4GB version.
You're being far too generous with pricing. Try looking up a 280x. I bet you can't find a good one for under $400 right now. Bitcoin mining has driven the AMD cards up in price about 33%. Also, 770s generally go a lot closer to the $330~ mark. I think the 770 is the best price/performance out of those 3 RIGHT NOW. If the 280x ever drops back down to its launch price, it will be a better value than the 770. But if you need a card right now, the 770 is probably the way to go. If you want a 290/290x, I'd definitely wait til the re-brands come out for better cooling, because the ref. versions often cross the 90c mark, which is insanely hot.