Sorry for the delay, as I wrote a couple of weeks ago the gearbox still skipped 4th. So I took it to my trustworthy mechanic.
First, it turned out that the rpm sensor was fine. The problem was that it didn't always skip 4th gear, sometimes but not always. But it also happened with engine turned off, key in MAR position, so he could replicate the situation and check everything.
After a couple of hours (he was very thorough) it turned out that the gearbox (manual part of it) is perfectly fine. The actuator is to blame. So he tried to find out what is wrong with it, unfortunately without any conclusive information. He switched places of the hydraulic plungers, cleaned everything and... the actuator works fine
. I've been driving it for 2 weeks now and it skipped 4th only once. This supports everything stoyce
suggested in previous posts.
So the problem is solved for now, I'm in the process of finding a working actuator right now for a reasonable price, you know, just in case.
Thanks for your help guys.