Holy crap, Batman, that IS weird.
My #3 & 4 are actually my coolest CHTs on climbout (JPI 730). Yours seems to be fuel related as evidenced by the higher EGT on 4.
By any chance does this happen around the time you bring the power back to 25"? If so, then the Economizer on your carb is doing its job well by giving you extra fuel at full throttle. Pulling the power back significantly decreases fuel flow. Try leaving the throttle all the way in to altitude.
Our induction systems are quirky and inefficient, to say the least. Airflow patterns in the induction tubes change based on power settings, rpm, air density, etc. My gut feeling is that turning on your carb heat decreases air density, which then enrichens your mixture, probably accompanied by small, subtle changes in deck angle and slight rpm changes resulting from airspeed increase. All of these things then change the airflow from carburetor through your induction tubes to your cylinders. These small changes then balance out your cylinders' mixtures which results in more fuel going to #4 which then lowers EGT and shortly thereafter CHT.
How to fix it? I don't know for sure.
It probably wouldn't hurt to use flaps on takeoff- or stop using them if you are.