Without any detailed specifications on exactly what type of actuator it is, I can only generalize.
Torque, given most every DC actuator I've worked with, will be directly proportional to amperage.
So, replace the 12V 4A power supply (guessing here) that current limits or shuts down under maximal load with a 4V 12A power supply (same power) that tolerates a stall condition for as long or longer than the actuator can. If the actuator is likely to fry if left in that condition for too long, protect it with a self-resetting thermal cutoff.