If you need to cool down even further, try to get a watercooling unit for that CPU.
well I've heard people going up to 5+Ghz, so at "just" 4Ghz I was hoping of not having to go water cooling, even though those look a little easier to install, but how do you overclock, is it a layman thing or do I risk destroying my computer??
Hm, I'm no expert on overclocking, but memory and mainboard are playing an important role here next to the CPU. The mainboard BIOS or desktop tools might have features to overclock your system. AFAIK you cannot destroy your CPU, since it automatically clocks down when it gets too hot. And before you destroy your system by overclocking, it just won't boot at all. You'll have to reset your BIOS then and start over (mainboards usually have a BIOS-Reset-Switch somewhere onboard, just check the manual). Just go easy on the settings while ramping them up.
I for my part wouldn't try overclocking. If you use this system primarily for rendering, the 4 Titans will do most of the job, right? Overclocking the CPU might be the wrong place to tune.
By the way: Packing 4 Titans in one case will give you a hell of a heat source. Be sure to cool them with additional coolers from the side (see picture below) or they might malfunction earlier than the rest of the system.
cosmos2_interior-side-panel_590x442-additional-fans von questargon auf Flickr
Without additional fans pointed at the Titans, the small gaps between them might be too narrow to suck enough fresh air in to keep the beasts cool.
At any rate: Monitor your system while working with it for the first time. I'll recommend http://www.cpuid.com/softwares/hwmonitor.html for the job. For overclocking, adjusting and monitoring your Titans, you might want to use MSIs Afterburner http://event.msi.com/vga/afterburner/overview.htm
Ninja Edit: http://www.youtube.com/watch?v=3vCrblKzKaM ^_^