In "Double Dipper", Dipper clones himself using Grunkle Stan's copying machine. He made his first full copy of himself that he called Tyrone. Together he helped Dipper Classic with his Wendy problem. When they overthought the situation, they made a third, a fourth (Paper Jam Dipper), another fourth, and then more.

The purpose of number 3 and 4 was to steal Robbie's bike so Robbie can get out of the way to Wendy. We never heard from them ever since even though Robbie returned to Wendy at the end of the episode. It is unknown why Robbie came back from the party. Maybe he gave up hope on finding his bike or he probably DID find the bike but with 3 and 4.

As far as we know, 3 and 4 are probably still in the forest or already disintegrated by water. They may have a purpose when Gravity Falls travels deeper to the end of summers if they are still alive. They could be important in a strange way, for the show has so many unexplained mini mysteries.

Also, Alex Hirsch better go along with the 3 and 4 fanfiction. If he doesn't, Soos will be very disappointed.