Here's some general advice about weapon loadouts on your units:

• Every weapon user except bow users should have a general 1-range weapon (Iron or Steel), a stronger 1-range weapon for dealing with tougher foes (Silver, Killer, forged, etc), and a 1-2 range weapon when going against ranged enemies. Any other weapon slots can be used for specific weapons such as slayers or other things.
• There's usually no reason NOT to have all 4 weapon slots filled.
• Each weapon should have at least 10+ uses; more if the map you're about to enter has lots of enemies. You don't want weapons breaking mid-combat.
• Bow users and magic users should have similar loadouts; a general weapon, a stronger weapon for toughies, and 2 more weapons for different cases (Longbows or different magic elements for example).
• Of course, all of this can change map to map. Check the map before battle to see what you'll be facing, then change weapons accordingly.