No problem. There are a lot of "Closed loop" water cooling systems that don't have a high risk of leaking as some people would expect. If it comes down to it, you can put silicon or a sealant around some of the tubing to make sure nothing leaks. These work really well under load, at least so far in my experience.
Air coolers still work much better than stock, the Hyper 212+ evo will probably be able to get you to that 4.0Ghz mark, or close. I previously used the Hyper 212+ (non-evo) with just 1 fan on it, and had my i7-920 OC'd to 3.5ghz from stock 2.66. It was stable, but load temps would go in the 80C's, so i brought it down to 3.0 and temps dropped to 70C max.