I had knee replacement surgery 4 weeks ago. I still have swelling and find it difficult to bend my knee.

Arnica is the best remedy post-operatively, to help with pain, swelling, and bruising. It is also called arnica montana. Get a 30c potency from a health food store. Take a dose (2-3 pellets dissolved on/under the tongue) about 3x/day. You can take it more often if needed to control the symptoms, or less often if the doses control the symptoms longer.

It is my sincere wish that someday surgeons figure out how much better their patients can do by giving arnica right away after any surgery. There are a few in my area that use it, but the majority are still very closed minded about homeopathy.

This should help within a short time.
Please feel free to ask for clarification if needed.

After arnica, it depends on symptoms. Once the swelling and pain are improved, if you have a lot of stiffness in the joint, or if it pains you in changing weather (like before a storm), then Rhus-t would be a good choice.
If the arnica stops working, and Rhus-t doesn't seem to be indicated (i.e. you don't have any stiffness, etc.), then I would try Ruta (which is good for tendons).
If the 30c is not lasting long, or if it helps for a while but then seems to do less and less, then do go up in potency (usually to a 200c). You can dose that the same way, but should find you need it less often...maybe once or twice a day.

You should also find that over days, you need the remedies less often as you improve, until you just feel you don't need them any longer. This may take even a couple weeks or more, but that depends a lot on your individual body's healing capacity.