These two sites suggest that there were only 3 bombs, with the names I gave above.
This site suggests there might have been another bomb, however, targeted for Sapporo, Japan, like M said. I was having trouble finding reliable information about this "extra" bomb (though I admit to only searching google for five or ten minutes), so if anyone has any sources, post 'em here, cause I'm interested. I read the book
Surely You're joking, Mr. Feymen, which in part describes Feymen's expirience working for the Manhatten project, so I find this stuff interesting!
Stephen
PS. There is the chance that bombs after the first three weren't associated with the Manhattan Project. Does anyone know when exactly the Manhattan Project ended, and when these extra bombs were made?