Prius Gen 3 Misfire Cylinder 4

My 2010 Prius with 200K kilometers has intermittent misfire in cylinder 4. I replaced all 4 spark plugs with new Denso iridium, Changed the order of ignition coils, cleaned intake manifold especially all 4 tubes within, egr pipe , valve and egr cooler which is a pain to remove and install. Also swith positions of fuel injectors after cleaning them. There is no sign of Head Gasket leak as no coolant reduction.
I had done everything what i could do. The misfire wasn't going away. It came mostly when the engine was hot and at very low rpms infact when gas paddle was not pressed. Took my car to several mechanics but no remedy.
Finally one mechanic said have you changed its transmission fluid. I have not done it said no. He recommended me to chane it see what happens. I opted and took my for a transmission oil change.
To my surprise the the oil was severely dark and only 2 Liters came out. I refilled it with Toyota ws fluid almost 3.8L drove my car and the misfire was gone. I drove 80 kilometers and no more misfire now the car runs smooth.
He said something that Prius has eCVT transmission which requires lubrication to cool the components.

 

@bisco you are right! I also thought this fluid change is a fixer. But my joy was short lived. It again started cold rattle in the morning.
I have noticed that coolant reservoir is loosing coolant but not overflowing or boiling.
So far what i have done
Change ewp
Clean egr cooler along with pipe
Clean intake manifold with all four cylinder egr tubes
Replace pcv valve.
One thing i have noticed that the radiator fans were running at high speed AC was off and the coolant temperature was 88°C
To my knowledge fans kicks in at 95°C
I hope its not the head gasket issue.

Copied for somewhere
"In 3rd gen the EER brings coolant temp. up faster to reduce emissions. Have had that part of system leak which has resulted in the coolant bottle boiling over from the pressure being introduced into the cooling system which then looks like a massive leak when it starts burning/ leaking into the exhaust system"
Is the above statement true.

In the morning i will open the reservoir cap and try to smell exhaust gas if there is any.
Here in my country those liquid color changing facility not available.
Can egr cooler also get leaked and gases introduced into the coolant?