Getting this achievement is simple enough - kill four enemies in quick succession with the pistols. Trouble is; your pistols only have one shot...

For this achievement, you have to get four pistols - which is to say, you have to craft four pistol holsters. You start with one holster, the second one you craft in sequence 3 memory 1. For the third one, you need two humpback whale skins - I found a humpback whale just south of Kingston; I've found that sea animals may move around a bit but there are general spawning points. I recommend upgrading your harpoons a bit before you go hunting however. The fourth holster you can craft when you've obtained two crocodile skins - crocodiles could be found in the swamps of Nassau. You can of course buy both the whale and the crocodile skins in general stores if you don't want to go harpooning/hunting yourself.

When you craft a holster you will automatically wear it and fit it with a pistol. Once you've got four pistols, getting the achievement is very easy. Just find a place with enough enemies (I did this while conquering a fort but there are more than enough other places that will work), and shoot with Y while an enemy is highlighted. once you've shot four enemies, the achievement will pop. Remember that you must "quick-shoot" with the Y button (which can be done as long as an enemy is highlighted); aiming with LT does not count as multi-killing (not even taking into account that it'll take way to long to keep aiming manually).

I really need to pick this game up soon. I'll definitely be going for this when i get my hands on Black Flag