The PlayStation Vita version of Borderlands 2 will not support 4 player co-op, a recent PlayStation Blog post has revealed. The developers haven't announced why it's been downgraded to two player co-op, but apparently it was very late in the game's development that the decision was made:
"Borderlands 2 is a blast when you’re flying solo, but the PS Vita version also includes support for two-player online co-op play between two PS Vita owners. We checked in with the SCEA Third Party Production team, who confirmed that while they strived to include four-player co-op in the game, they made a decision late in their development cycle to focus on providing the best two-player-only co-op experience possible. As a result, the maximum co-operative player count for Borderlands 2 on PS Vita is capped at two."
Good news if you played Borderlands 2 on the PS3 and intend to buy it on the Vita too, however; you can carry your save over, you just have to upload it to the cloud. This only works for the standard and GoTY editions.