UPDATE: Found that quest will complete if I have all 4 limbs when talking to him. However, it didn't give me the opportunity to choose which limbs to give him. I had more than 4, and it just gave him the first 4 in my inventory, which included a Living Stone Leg that I didn't want to give him.
SUGGESTION: Allow the player to choose which limbs to give him so they don't inadvertently give Faust valuable limbs they're saving for themselves. Also, allow the player to start turning in limbs even if they don't have 4.