The system of four slams is great. It has slams at the four most traditional tennis countries. Also, there are four different surfaces (even though they should be a bit more different).
If there would be a 5th slam, it should be on fast indoor courts, the biggest indoor event is WTF. But WTF isn't a slam, maybe it would be more slam-like if it had a field of 128 and Bo5 (would be hard to have under one roof), or if it were a joint event.