Musclenerd unlocked the new baseband. The 3.x basebands are incompatible with the 4.0 firmware. You can't have the 4.0 firmware with the 3.x baseband.
At least that was the case with the 3.0 firmware and the 2.x basebands.
That is not correct.
I am running iOS4 with 5.11.xx baseband. It works perfectly.