Hotend thermal runaway

Same problem for me. 
I have rebuilt my MK4 to MK4S, and when doing the tests after the Hotend heater test didn't pass. I tried a lot of different things with no success. And finally one of the heater cable broke.
But the superb Prusa support sent a new heater and thermistor free of charge! 
This solved the test-problem. But now when I have started to print I've received this error.
Reducing the fan speed to 68% seems to solve the problem for me as well. But a bit annoying. 
(I'm still running without the sock) 
/Per 

Just wanted to share my experience with this error.  I had the original MK3S (kit) and upgraded that to the MK4.  I've never had any errors on those two iterations.  I then upgraded to the new MK4S (with new extruder parts, high-flow nozzle, etc).  During the assembly the instructions relating to the "Silicon Sock" (which was new to me) said that it was Recommanded but not required.  I chose to install it as it was recommended.  When I powered up the machine (with firmware updated to latest), i went into the Settings -> Hardware and changed the machine from MK4 to MK4S and also "checked" that the Silicon Sock was installed.  During the initial calibration/test, the Heater test failed.  I tried it several times and each time during that test, when the nozzle got to 178 degrees C, the heating test would fail and then begin cooling down.  So, I decided to see if the sock had something to do with that.  I removed the sock, i then disabled the setting that said the sock was installed.  Then ran the heating test again:  this time it succeeded.  So I decided to keep the sock off.
Next test of this new printer:  printing of course.  I updated prusaslicer to 2.8.1, downloaded any configuration updates.  Went thru the config wizard and picked MK4S with HF0.4 nozzle. 
Every print I started (generated via the new configuration in PrusaSlicer) would failed with this Hotbed runaway error in exactly the same fashion (multiple prints... note this was PLA):  The bed layer would print perfectly at 230C without any variation of temp (note that the big new fan is off during the bed layer).  When it began the next layer, the new fan would kick in, the temp would get told to go down to 225C (in this particular case) but the temp would actually dive down to about 212-215 and then slowly start creeping down lower and lower until this error occured.  I tried this over and over and exactly the same thing on any print.  I even changed the profile in slicer to the old MK4 just to try it.   Same thing exactly.  
So, i decided to install the silicon sock again.   This required, me removing the nozzle/wires, putting on the sock, re-inserting the nozzle and hooking the heating wires all back up.
I now have no problems.  The temp is perfectly stable.
So either:
1.  The sock is actually required in order to maintain temp better with the new fan/shroud??
Or (more likely I would guess)
2.  I had some cables pinched or crimped or something with the nozzle not quite installed correctly and removing the nozzle and re-installing it fix things.
Anyway, just wanted to share as that was a several hour support chat.

Same problem here. Once the temp started falling, I reduced the fan speed to 68% and no error after this. Kind of annoying to watch the print and adjust the speed manually each time.

My MK4 jeeps doing this. I have gotten this error 4 times in the past 2 days. I had this issue before, about 5 months ago, and worked with support to fix it. We took the whole heatbead apart and that somehow fixed it, but now it's happening again. Does anyone have any ideas on how to fix it? 

I started getting this error after installing a different fan shroud from Printables, seems like the new air flow made the thermistor miscalculate the actual temperature of the hotend.