4 weeks after arm lift. How come I can't stretch my arm well?

Good morning and thank you for your question. It is not uncommon to have tightness after arm lift - after all, surgery is performed to tighten the upper arm area. In addition, a period of limited movement during the post-operative healing period can make the arm stiff. So once you are cleared to resume normal movements of the arms, it can feel very tight. In most cases, this gets better over time. I generally instruct my patients on a regimen of range of motion/stretching/strengthening exercises. With that said, any activity that causes pain and discomfort is the body's way of telling that you're over-exerting yourself. Please contact your surgeon to discuss this issue as well.

Any activity that causes pain and discomfort is the body's way of telling that you're over-exerting yourself.  While it may feel that you have already bounced back from your surgery after about a month, take note that what you described is not uncommon.

During arm lift, the loose skin is removed and the remaining is tightened and reshaped.  The fascia beneath is also tightened to achieve a smoother, more natural contour.  Hence, it might take a few months for the "tightness" to go away.  Nonetheless, a meticulously performed surgery will not in any way affect arm movement/rotation.

However, in order to really allay your concern, your original PS is in the best position to explain to you the possible cause/s of the discomfort and pain you're experiencing. 

Good luck and best wishes.