Are you asking about the pros/cons of 2 in the rear versus 2 bags in the rear and 2 up front? There is no con with starting out with 2 in the rear since you can always add 2 in the front later. For riding around town, 2 in the rear are plenty. For touring, you may find that you'll need 2 up front. Plus bags up front on a low rider will help improve the handling of the bike when touring.

Or, alternatively, you could go ultra light and use only 2 on the rear. It depends on your style of touring.