I´ve said this many times, and it seems now that it wont be implemented, but...
PROPOSAL:
You should be able to have at leat one extra slot for weapons, maybe unlocked after you get certain perk, certain encumbrance level or maybe (if you are in a horse) after you are able to buy horse armor.
It is ridicoulus not be able to fullfill certain weapons combinations because four-slots limitation.
Imagine you are a horse archer. You are on a horse and you either have small range melee weapon or long range, but you cant have both because you have already spent two slots on bow and arrows. The solution is going out there without a shield, which is even more ridiculous.
Imagine you are a footarcher in a big army. Your main objetive is to keep yourself alive and shooting, most probably behind your infantry. It will be more likely that you should have not one but slots dedicated to ammunition (bolts or arrows). Well, that leaves you without protection (maybe no shield) or without any melee weapon, not even a dagger or knife.
It is clear that usually you dont need a dagger or knife in M&B because game mechanics dont reproduce well close fighting, which is for me the greatest weakness of this game. But after all, to give a closer feel to real battle gear, you should be able to carry a small backup close range weapon. Even the lower soldier of an army will have a knife in his belt. Well, if you are a fighting peasant in M&B you cant... one slot for sickle or chopping axe, one slot for a bag of throwing stones, one for a makeshift shield of some sort and one for a crude staff. That leaves you with no place for your eating-close range combat knife.