At four weeks you probably still have lots of swelling. Is that right? If so, the swelling will absolutely stop you from bending your knee. If swelling and pain are still bad, do icing four times a day and keep elevating the leg above your heart while you're icing. Don't try to force the stretches. You will end up causing more swelling and pain if you do. Range of motion comes and goes. It isn't true that you have to reach a certain degree of bend within a time limit. Everyone heals differently. If you have ever had a broken bone and had to keep the limb immobile you might have experienced frozen joint, where it's too stiff to move at first after being immobile for weeks. But with gentle therapy you can get the motion back. Same for the knee. Let the joint heal.