What are the speakers in the 4x12 rated at? What is the cabs impedance? What is the Heads impedance? Given that you're playing a guitar into an amp that's rated at 60w (conservatively), and are using a cab that is probably rated at over 100w, no, you're not going to blow your speakers unless you start pumping down tuned, bass heavy, distorted sludge. Just be smart, and listen to your speakers, make sure you're not over exerting them. And don't mismatch the impedance.
