Is there a Super Saiyan 4 Gohan?

No, not in the authentic series, just only in fan made ones. In the actual story, only full blooded Saiyans can turn into SSJ 4. There are knockoffs and side stories made by fans and unofficial groups, but the real Dragonball, Z, and GT series never feature him surpassing Super Saiyan 2 aside from unlocking his potential. There may be pictures of him as SSJ 4, but the series never mentions it.

There could have been a Super Saiyan 4 Gohan, but since he doesn't have his tail anymore, he can't turn into an Oozaru and then Super Saiyan 4. If Akira Toriyama did come up with the Super Saiyan 4 from earlier, he would have already in the movie: The Tree of Might.

Only pure Saiyan's such as Goku, Bardock, King Vegeta, or Vegeta could turn into one, although only Goku and Vegeta used it in the TV show. It was never revealed if King Veteta or Bardock could use it. we can only assume that they couldn't because if they could it would done them a lot of good to use it against Freza before he killed both of them.