I had the same problem a few weeks ago. I soaked all four bolts with penetrating oil, which didn't seem to help at all. I finally bought some longer allen wrenches (they should be metric). I got three of them loose, but that last one got very stripped out.
I ended up using a cold chisel and a hammer to make a "notch" in the edge of the head, then gently tapped the notch with the chisel at an angle, from the side. To be honest, I was surprised I got it out at that point. I thought I was going to have to drill it out and use an EZ-out, which is never a sure thing.
I replaced them with hex head bolts with Anti-sieze on the threads, which should make any future maintenance a little easier.
Good luck!