How can I lose 20lbs in 3 of 4 weeks?

I just had surgery on my leg so I'm kind of restricted from walking as much as I used to.

It is still possible to lose weight without being able to exercise. You just have to watch what you eat, and be very dedicated to your diet. Portion control is key. Along with minimal carbs (breads and pastas) minimal amounts of fat as well.
You should also try watching you calorie intake. Lets say for example you weigh 200 lbs. to find the amount of calories that you need to consume a day to maintain your current weight, multiply your 200 lbs by 10 to get 2000 calories a day. You will need less calories to lose weight

Try drinking lots of water, it should improve your metabolism and make you fell less hungry between meals, I also with holly portion control is most important stick to 3 meals a day but don't starve yourself it will make u gain weight.
Try upper body exercises if possible don't push yourself too hard