How do you use your four weapon slots in Far Cry 3? I own the holster/sling for four guns but I can only use two even though I own more.
Go to a shop, and then go to the weapon you have (or want to buy), and then select the 'Equip' menu:
Then, assign that weapon to a slot:
You should now be able to switch between all four weapons, assuming you have assigned weapons to all four slots and have the Heavy Duty Holster (four weapon slots).