Backstab does much more than 4k. As I have just learned the hard way. 8k hits from stealth are very much possible. Pair it with a heartseeker that you cannot see because the game takes ~1.5 seconds to render their model and you've got a recipee for fun.
Disclaimer: The above poster may have have been outskilled by a thief and is using a forum post as a means to vent his frustrations. Take all messages therin with a grain of salt... Get it... salt... This guy salty as hell.
Edit: Fix stealth render lag. Is really annoying