4 Amps is peak, I have never timed how long it will take to damage one, but probably not long. Attaching a heatsink will help in that case.

The H-bridges from robot power look like they will work, but you will need a circuit in between the Serializer and the H-bridge to take the PWM and Dir signals and turn them into the proper signals. You should be able to do this with 1  7402 IC per H-bridge. Something like this.

Be carefull putting a large cap across motor terminals. A cap like that is polorized, and if you reverse the motor diection it can explode. Not a knock down the house kind of explosion, but a (why is the cat hiding in the closet) kind of explosion. I would not want to be holding onto the cap when that happend.

As for my H-bridge, I use an International Rectifier IR2184 to dive the H-bridge. It requires 10-20V to run (or it will definitely not run). I put a 12V regulator on board so It can be used at 24V, but that means it drops a volt or so at lower voltages, so if you put 12V on it, 11'ish will get to the 2184 and it will still run. I don't think it will work at all below 10.

Ringo