Sounds like the syncro is worn on 4th, it does get some hammer as its the lowest high range gear before needing low range. I would imagine the existing driver has got used to it and may double de-clutch to engage it. Only thing you can do is change gear like a proper trucker, you will only get this gear if you match the engine speed and road/gearbox speed. If you try to rely on the synchromesh to engage 4th you won’t get it to engage easily, does it get slightly better when the gearbox warms up? On a 14 year old truck the box is bound to be worn, thats a lot of gear changes!
Try a bit of double de-clutching, its an art form if you do it well, the benefits of using this technique are useful, its faster to engage a lower gear and improves control but its not easy to learn for some, frankly i’m a real geek for proper old school driving and I can do it very well but some drivers are hopeless at it. Have a look for it on youtube, I know the modern gearboxes don’t advise this technique but thats only because they don’t think the average driver is capable of it.
Goes like this, 5th gear at 1200rpm, clutch in and shift to neutral, stab the accelerator to punch the revs upto 1500rpm and then depress the clutch and shift it quickly into 4th, no half measures, do it confidently, firm but not harsh. You won’t get it every time but it will start to feel good when you get the hang of it, sounds good too, especially if you engage the exhaust brake just after finding the gear! If the vehicle surges as you engage the gear you raised the engine speed to high, its normally around 200rpms between 4th and 5th, so if your at 1300rpms and need to go down one full gear take the revs to around 1500rpm to engage the gear, or in other words - learn your engine speeds for different gears at different road speeds. Like I say, you need to drive the truck properly now, those driving lessons don’t prepare you for the real world - ie worn out scannies!
The engine speed needed in different gears at different road speeds are often in the truck handbook, but you can learn it buy looking down at the tachometer (rev counter) just don’t take your eyes off the road for too long! Good luck. I love driving worn out gearboxes, my scania at keedwell had a worn out syncro on 5th (8 speed box), but I got it in most of the time by using this old school gear changing method.
Also if you want to know how to use worn out gearboxes, ask a coach driver, as most old coaches are pretty well worn. No point asking the boss to get the gearbox reconditioned as they won’t spend the money on it, last one I got repaired was £3000 on our Dennis coach, had to get it done because customers expect an easy to drive training vehicle.
