A fresh xray is needed to see if the fracture has healed or not now. Comparison with the post injury xray will make it clear as to whether healing has occurred or not. 4 weeks is a little early to remove the plaster. Ideally bone takes about 6 weeks to start joining up. For it to become strong like normal bone it will take even longer. Must likely the fracture in your foot is healing but pain may be present due to incomplete healing process. Swelling after removal of plaster is common. It will subside with time. I would suggest you ready your foot and not walk around to much. If pain still persists them visit the ortho surgeon to obtain a fresh xray to see if it has united or not. I cannot prescribe meds as Practo prohibits me from doing so.