M4 and M16 in airsoft only means a difference in barrel length and the type of stock (some times not even).
The A annotation you can think of as different versions; each with slight modifications that may be beneficial or detrimental depending on your personal preferences. (eg stuff like a detachable carrying handle).
Since you're buying a M16 length barrel for an M4, you'll need to get either a longer outer barrel with the matching longer handguard, or you can do it the easy way and get a silencer.
Depending on the brand of your current gun, you can just get a non-ported cylinder, and not the whole set.