4 days post op. L knee arthroscopy. Alot of pain on outer side of lower thigh. Is this normal? Thankyou

Yes. This can be from swelling inside the knee, a tounequit used during the surgery or from a clamp that is used. After this type of surgery, pain is usually worse days 2-4. It will improve.
Let me explain. Most likely you have what we call lateral release for knee cap problem, if that the case it take a little time, it is not check it out.
Knee pain. As I am not your treating physician i would contact your surgeon's office immediately and have an appointment so the surgeon can assess the portal site and ensure that there is no infection.