dont underestimate how hard it'll be to get stable much past 4-4.2GHz... with water, you'll be able to do it, but you might have to play with the voltages quite a bit... 4GHz should be no problem to run 24/7, you might even be able to squeeze out 4.2GHz, but dont expect to get really stable at much beyond that... suicide for sure, you'll be able to crack out some insane pi scores at 4.5GHz+, but not much else
oh yeah... what kinda voltage are you puttin through that thing to get 4GHz, nos? most of those chips should be able to do 4.5GHz with <1.5V... actually saw a 4.4 run with only 1.39V in the BIOS
EDIT: the highest clock i've seen on water anywhere on the net is 4.65GHz (vcore was 1.55V btw)... thats a base clock of 221MHz, which is really stretching modern mobos... in 6 months or so, we'll probably see BCLKs around 250 or so... but that was just a suicide run, so you might be able to hit that speed for a pi run nos, but you'll have to really work the QPI for that one