Something weird developed on my Multijet - when I'm accelerating hard and I shift from 3rd to 4th at approximately 3.5k RPM or above, I get a weird feeling on the lever as if it has some trouble getting into the gear. It still shifts let's say normal, but I can feel slight hesitation on the lever. It doesn't make any special noise or anything like that, only the feeling on the lever is different.

Now I know what your first thought will be - I'm trying to shift too fast but I can assure you that's not the case. I've had the car for 2 years now and this is something that recently developed, regardless of my shifting.

The gearbox works perfectly normal in all other conditions - shifts normally in all other gears, doesn't throw into neutral etc. It goes into 4th without a problem when engine RPM in lover. I also changed the oil last year.

not sure if related but when I was not driving much a year or so ago due to an injury the lever seized slightly and wouldn't self centre causing me trouble usually 2nd to 3rd when I started driving again.
I searched for possible causes and found corrosion on a shaft rotating in plastic top hat bushes at the gearbox end of gear cables were the likely suspect so without being able to do more(due to injury) I had to make do with spray penetrating/lube oil on cable ends and liberally over the housing that had the bushes in(did several times in a week to make sure it got into the shaft) and just to check also lubed the plastic ball joint of the gear lever under the gaiter.
the more I drove it again the better it got to the point I forgot about it.
as I said may not be your issue as 3rd to 4th is a straight pull back, you could do worse than lubing the cables and pivot points anyway, whilst your checking for other issues.

Thanks, might as well be something around the linkage (well, at least I hope it's not a syncro since those repairs tend to get expensive).

I forgot to mention another thing - when shifting at lover RPM, you can slam 3rd to 4th all you like (faster than when shifting at high RPM and feeling that annoying resistance).