4 days after left knee replacement surgery, i've just noticed a blotchy red rash on my inside upper left thigh, also some bruising by the wound ?

Call Dr in am: Let the surgeon know what you are experiencing. He/she will advise if you need rechecked sooner than planned, or can try remedy at hoe.

That is quite normal: due to deep bleeding from the knee gravitating to the thigh, by being in bed & keeping the knee elevated. Similarly the bruising around the incision itself is generally due to the bleeding s well. This will resolve by itself in about 3 weeks, unless you are on Blood thinners then it might linger for 6-8 week. If any increasing symptoms like fever, chills or pain, then see your surgeon. Good Luck.

Rash: Post operatively if you notice redness in and around the surgical site should be evaluated to rule out infection.