rich mixture in the fourth cylinder
 

The car is 1984 carrera targa. European version. The fourth cylinder gets more fuel from the enjector than other cylinders. What could be the reason? The enjectors are cleaned two weeks ago (filters have been changed also). Ecu? Relay? valve setup?
thanks in advance..

Are you "reading" the plug? Could be a bad plug or wire.

Leaky injector? See if you can isolate the problem. Swap the injector to another cylinder and see if that one runs rich.

They are all checked, changed and tried etc. I dont know how the service realized this but they said that they put it (ECU) to the leanest setting but still the mixture is rich.