I just can't make this down-change! It is just like there isn't an available "slot" to shift into! Up-changes are O.K., it's just that particular down-change. So what's gone wrong and how do I put things right?
Worst case - I had exactly the same problem from 5th to 4th and it was a worn 4th gear selector ring - that needed the gearbox out and stripped to rectify. Fingers crossed it's a cable adjustment fix...
Yes, I've done this (cable adjustment) No change (that's a pun!) So changing down from 5th gear to fourth just hits a stop in the middle position! It is only that single action that is a problem. 4th gear can be selected upwards from 3rd. If you "jerk" the gear lever with some force, from 5th to 4th, then it can sometimes be selected.
So are there any further suggestions?
Have you tried double de-clutching and rev matching?
If this makes it work, then we can fairly confidently blame the synchro ring on 4th. If it makes no difference, then the shift tower becomes a prime suspect.
The 2nd gear synchro ring in my gearbox has been worn for a while, but because double de-clutching and rev matching is my habitual way of changing gear, it's of little consequence.