I just had my MacBook Pro "repaired" for this issue (excessive heat and noise when 4k external monitor is plugged in). I explained to apple support and then they shipped me a box for the "Repair". I had to ship it to apple since all stores are closed in AZ. They swapped out the entire logic board, graphics card, CPU, and Touch Bar. I'll let you guys know if this fixed worked or not.
following up -- at first blush the "repair" did not work