4ghz definitely won't be a bottleneck! Anything over 3.2-3.6ghz (depending on the gfx card) is pretty much extra gravy right now
Good luck with the build! And one more thing: if you haven't already, google around for ways to apply thermal compound to a direct touch heatsink. It's a little different. I put the past between the pipes first, which I think is what you'll find many suggest. But like I said, google for it so you can look at some purdy pictures too!