Hi,
The transmission on my 124 Coupe has recently started jumping out of 4th gear. I've stripped the transmission thinking that the 3/4 selector fork must be worn but there seems to be no more wear than on the 1/2 fork.

If there is ANY wear on a fork, in my opinion you should be changing it. I would also suggest you change the related slider and the gear it is popping out of. Unless you don't mind doing it again if it did not fix it, or you don't mind fixing it again soon because it has prematurely failed, again.

There is a little bit of sideways movement where the fork engages on the transmission slider but, again, no more than for the 1/2 fork/slider. Is wear of the fork/slider the most likely cause of the transmission jumping out of gear? If so, do I need to replace both the fork and the slider or just the fork?
Thanks, Brad

It's not necessarily caused by wear on the fork/slider. Wear on the gear and fork often cause it, then the fork and slider wear as you try to keep it from popping out. I say Change it all!

Are aftermarket forks as good as OE (what can you supply?).

In forks we don't get a choice any more. But if you want the transmission to work better/longer, always use OE parts if they are still offered and you can afford the additional expense.