I do not see any problems with the current system. It allows freedom, and causes players to think what it is they would need in a fight. Personally I don't use shields, just so i can carry a lance, a 2hander and a bow. (find it most optimal since I can do couched lance when on horseback, shoot from far, use 2handers when close range for more speed and if my poor horse gets killed 2hander is alot better than lance)
Somebody else may want to use a sword with shield, a lance with shield, and a mace with shield (for 'bring prisoner' mission, so they can slash, stab, and bash when nessesary)
Others may wish to go full time archers, so they would carry a bow with 3 stacks of arrows, or 3 quivers. (I'm sure they don't actually hold the 27 arrows in their hands)
Variety, very straight forward and easy to use, and at the same time lets you decide what it is you need. I mean in real life, if you have a quiver strapped to your back, but remove it, you can strap something else in its place right? So having additional slot will either remove that possibility, or give an extra quiver, which is just as well adding additional slot. I'm against the whole idea personally...
As for the current 4 slots, I believe that 1 is on the back, 2 are on the belt, and the last is in your hand :

so nothing unrealistic or impossible about that in my oppinion :
