Actually, 5. a4 is a great move because it breaks up his pawn structure. Note that if he plays b4 you can move your knight to a2, attacking two pawns at once (b4 by the knight; c4 by the bishop). He will likely play b3 when you can move your knight back to c3. Your bishop would still be attacking his c4 pawn, which he will lose because he cannnot defend it again.
A move like 5. a4 is called a Minority Attack because you are attacking a pawn majority with a pawn minority in order to create weaknesses.
This started as a queen pawn opening that went wrong so I chose to give material and take lots of unpleasant risks for better position. I hope you enjoy it.