MacBookPro (2015model) fan issue when connected to external 4k display

[Nanoporous]

This issue has been a while. When I connect my MacBook pro to an external display, the fan goes up crazy. More than 6k rpm:

I am not sure this is normal. In fact I have a vague memory of connecting it to 4k display without this much fan noise before.
Any hints/suggestions of what could be wrong with my machine?

 

[Audit13]

Driving an external monitor at 4k will generate more heat. Doing so while running graphics intensive programs will cause even more heat.

 

[Nanoporous]

resetting SMC seems working, now the fan speed drops to 4000 which is far less annoying