That is because below 50 PVP all your skill and stats are scaled temporary to that of a 49 for the duration of the WZ. So by using that fact, what you should be saying is "I am hitting 4.3k Ambush at lvl 49"; which is quite pathetic, actually.
Sharpshooter spec Gunslingers is all about burst as all their core abilities triggers Trickshot 100% of the time and the buff last about 4.5 sec. This give GS a lot of freedom in how much they want to burst and when to do it.
Stop looking down on him, it's not like that at all. Level 13 has literally no secondary stats on gear, i.e. crit/surge etc, so yes, 4.3k is pretty good.