Idling problem:2002 Polo classic 1.6 AFX engine

[Smash Afrika]

Greetings,
I hope everyone is well.
So I have an issue with my car, when it's cold and I start it it idles just fine ,but after driving for a while ,like 10km or less,it starts having a fluctuating sound and the rev count moves from up and down ,just between 900rpm to 1000rpm ,with 1000rpm being where it is on initial start ,
Service history:spark plugs (NGK), spark plug lead set,
Oil,petrol and air filter,new oil ,new coolant)

Did compression test:
Results : cylinder 1,3 and 4 =100psi
Cylinder 4= 85psi (15% lower than the other 3)
Poored oil,it still happend,cleaned throttle body,still there,
Note,it doesn't seem to affect power as the car performance is just fine ,

Any suggestions or anyone who's had a similar issue?