96 td27 and 5 speed manual problem

Hi guys, I have whats called a Nissan Atlas box truck and it has a td27 and 5 speed manual. I seem to have lost 4th gear. It started feeling like the gear selector wasnt going far enough to fully engage the gear so I looked underneath and noticed the rubber on end of cable that the bolt goes through was split so I fixed that.
Still no go, cannot select 4th at all now, tried adjusting the cable but to no avail.
All other gears are fine.
I took the 3rd and 4th cable off and selected 4th by hand but still just crunches and wont go into gear.
What can I try next please?