You came to the right place!
Make an empty array with 4 places (if you have four weapons, for example), i recommend using arraymaker so you can make an arraymaker proxy, you can find it on ecosystem.
Put your first weapon on index 0.
When you pick up another weapon, according to it's type/name or whatever you want to categorize it with, put it to appropriate index in the array
When you switch weapons (get key down), get the appropriate object/reference from the array
Equip that weapon (play animation, enable it on the player character if you're actually having them all on player and just disabling them and enabling them as you pick them up)
The answer is a bit general, but bear in mind your question is a bit wide and we don't know the specifics of your setup.
Cheers!