The 66A rating of your MosFet could be misleading you. That rating is contingent on the FET being bolted to (with a very low thermal resistance) to an INFINITE Heat Sink.
Also, you didn't specify if your circuit drives the FET as a switch, or as a gate-voltage dependent variable resistor. If used as a switch, your unheatsinked FET might switch your 4A (or it may not)! If you are using it to vary the current through the actuator in an analog way (non-PWM), then you WILL have to bolt it to a heatsink.
Also, be sure that the 0 to Vcc voltage swing on the PIC port pin will fully turn on the FET. Some FETs require 10V on their gate to fully turn them on.
OOps, I was posting as you were. Your circuit will turn on the FET hard enough. Why not just use four FETs in a H-Bridge, and get rid of the relay...