Pokemon Fire Red Question?

I've found the move deleter in Fuchsia City. If I take a pokemon and delete a move so that he only know 3 moves and then I take him the the daycare place and while he's there and learns a move will it fill up the blank space or will it be dumb and probably replace a move that the pokemon already knows? I'm pretty sure it would just add the new move in the fourth spot but I just wanted to be sure