I can tell you from experience it's not guaranteed. If you're using it on a pool or coin that does frequent work restarts, like p2pool, you won't get 2,000 GH/s. Just like the S2, it does not like regular work restarts.
M
Hm, my average is 1.8863636364 TH/s over two weeks. Tried on several pools. So it is 0.1136363636 below guaranteed hashrate. With 6 S4, this means that I loose about 20 TH/s per month. So if there would be a guarantee, they should reimburse me for this :-)