Sorry you haven't had any replies yet - how are things going now?
Have you tried putting him down earlier than 4pm? I wonder if he's already OT by then, especially on the days he only naps 2hrs. The average A for this age is around 3 hours, so if he naps 10-12, I would be trying to get him down again around 3. Or do you think he's generally low sleep needs?
He definitely needs two naps at this age anyway, and for the next 3-4 months at least, so if you're really struggling to get him to have a pm nap, I would cap the morning nap to 2 hours, and try for another 3hrs or so later. What do you think?