I ran 2,3,4's.. In my old 421 i went from the 2's to the 4's.. Honestly didnt feel a power gain or a power loss. What i did feel was that the 4's seem to be a bit more responsive over the 2's when you stab the throttle! Id say for 200 bucks just get the 4's! Doest make much sense to me to buy used 2's or 3's for (usually) 125-150 when you can buy new 4's for a couple extra bucks!