The Corsair H50 isn't as strong a performer as its replacement, the H70. And both are outperformed by the very best air coolers.
You don't say who built it, nor who will OC it. If it comes to you OC'd at 4GHz, you can stress test it yourself, and quickly answer your own question. Same if you are OCing it yourself. You'd use Prime95 and CPUID's Hardware Monitor to watch the temps.
personally, if I were speccing a build to try for that overclock, I wouldn't have used the H50. I would have used an air cooler. I don't like the self contained water coolers because air still performs better, and your system can't tell if/when the pump fails . . . other than by overheating.
You can give yourself a little breathing room with a WC like that and install something like CoreTemp or RealTemp...they both can shut down a system with a shutdown script once temps reach a certain threshold. It doesn't make it any better; only allows your system to go down sooner if the pump dies (which, I wouldn't trust one of those H50/H70's with anything I owned) and saves you from heat. Even so, most newer chips are equipped with thermal shutdown anyway.
Unless someone has $300+ to spend on watercooling, I don't even recommend these 'single-box' cooler solutions in any configuration. You are better off with a high-quality air cooler in every situation; and they perform on par or even better than the H50/H70 depending on fans used.
So, to answer your question of 'can it handle the i7 930 clocked to 4ghz?'
No. Not like it should for the money spent and temps you end up with.
But also, why do you feel you need 4ghz? I mean, it's fun to join the 4ghz club but as a 24/7 OC it's generally too high without a really solid cooling system, but even then, you can get far better efficiency at a lower clock and run with Turbo boost and all the power saving features enabled. I mean if we're talking gaming, on an i7, anything over about 3ghz isn't really going to speed things up much. You can easily do, oh, 3.6ghz and have a ton of headroom in gaming applications. My i5 is at 3.7 with turbo on and the usage in gaming is rarely above even 50%.
^Yeah, in most cases, the 'x.x ghz' moniker isn't all it's cracked up to be. I've had my Q6600 G0 at 4.0ghz before, but it's not stable at CPU load over 75%. I typically keep it between 3.2 and 3.4ghz and it's rock solid and I've never had any issues with any application or game...ever. Yeah, you'll score better in benchmarks, but in real world applications, where does this benefit you? When you are getting 80+FPS in a game...is 85FPS going to make a difference? Not likely.
Yes, its always fun to push the limits and its good to learn how a system works, so I am not knocking your efforts...at all. If anything, I applaud them. I just don't think that everyone realizes that because they have read 'so and so's post about their clocks on a similar system' doesnt't mean you also automatically can achieve the same results on yours. Each hardware component is different, almost like people are different, in specific capabilities, environment, usage and lifespan. Yes, you can expect 'similar' results; no you can't necessarily guarantee them every time on every similar component or build.