I have recently had back surgery for the 4th time. What is best to restore strength in my back?

Exercise . After this many operation it is very hard to keep up. Go to therapist and have him a home program of exercise and follow them. Happy new year.
Compliance. If this is your fourth surgery, you would certainly want to be very compliant with your surgeons recommendations to decrease likelihood you will ever need surgery again. That being said, once you are cleared for it, a core strengthening program would have the goal of strengthening your paraspinal muscles.