This is fairly easy once you craft the pistol holsters. To get the 3 additional holsters, you will need 2 Ocelot Pelts, 2 Humpback Whale Skins, and 2 pieces of Crocodile Leather. After crafting all three, simply open combat with 4 guards and use the in combat y button to quick-aim and kill each guard. Once in combat it is a good idea to put a little distance between the guards and yourself. It may also help if you get a civilian to grab one or two of the guards for you.
Additional info/Alternative method (credit to Pablo315)
This need not be done in open combat. It can be done with 4 guards just standing near each other as Pablo 315 found in Havana.