We can get the pistons free... if you don't want to spend the loot then call and I'll give you a bunch of tips to help you get it done. Here's a couple simple ones:
1. With the cables off, tap the handbrake arm with a rubber or plastic mallet or softly with a hammer. This will force the inner piston to move breaking the rust bond. BE CAREFUL... look at the side of the arm. There's a stop post there that you don't want to break off. Watch where that arm will travel before you pound away too hard.
2. Turn the inner adjuster with a high speed air wrench or drill (clockwise). This will "usually" get the piston moving.
3. For the outer piston, take the caliper apart and position the outer caliper 1/2 in a vise piston down. Remove the 13mm plastic cover (if there is one.) Remove the retaining clip. Remove the 13mm lock nut. Now, tap the shaft of the adjuster down with the plastic mallet again to break the bond and move the piston out. Now use your 4mm hex to (counterclockwise) bring the adjuster back up. Tap it down agan, further forcing the piston out... rinse, repeat. DO NOT attempt to use the 4mm to turn out this piston if they are stuck as you say. This will only strip and ruin the adjuster. Tap down with a plastic mallet, back adjuster out. Tap down with a plastic mallet, back adjuster out. Tap down with a plastic mallet, back adjuster out until the piston and ajuster fall out.
You may want to buy a can of Gunk carb cleaner as well. Recent oil prices have made a gallon of this stuff North of a Jackson but it's worth it. Soak the calipers in there for 3-5 days.
Enjoy.