I have absolutely no idea what the problem is with your brakes, but I will tell you that with my four piston Sierras up front, which are pretty similar to the four piston Wilwoods, and the outboard braked rear behind me that uses the floating PBR calipers that went on Corvettes and the performance Camaro/Firebird builds, that I can lock my wheels up without too much trouble. Now, I did switch to Hawk street pads to eliminate the squeal/groan of whatever pads were on there to begin with. Wish I could contribute more, but I just don't know what the problem is.
Do you have any inclination to move up to what ERA calls the "big brake" option? In my experience, when you can't figure out what the problem is, if you just start replacing everything in sight with bigger and better stuff you will eventually stumble on whatever it was that was causing the problem and you will have fixed it.
