Yaris won’t shift into 4th gear

[Matej748]

Hi 
I have a Yaris 1.4d 6. Speed manual
Please, has anyone encountered the problem of not being able to shift into fourth? At first it wouldn't shift just when understeering, so I put it through third, but now it won't shift at all. 
When I'm stationary whether the car is started or off I can shift it just fine, but when driving I don't have a chance even if I push it harder. But I have noticed that when I slow down and get to 20km/h then I can get it to shift there.
I'm thinking it might be the synchro and I don't know if it's even worth remanufacturing or getting hold of a used one. If it is the gearbox then. The oil in the transmission is changed.
Thanks for the advice.

[flash22]

mileage (KMs) ?

bad lower engine mount (dog bone)

when was the last time the gearbox oil was checked/changed, have you dropped the gearbox fluid and made sure there is no glitter in it

[Cyker]

Might be worth getting the gear linkage cables checked and recalibrated too - On the petrol Mk2 Yaris with 6-speed gearbox it's a known issue where it doesn't slot the gears in properly after a while which causes premature wear.

[Matej748]

The transmission oil was changed 3-4 weeks ago, and the Yaris has 210000 km

[flash22]

If there was nothing in the fluid its either linkage or clutch related, i take it 2nd and 6th are ok

[Matej748]

Do you think the clutch does it?
Because the other gear go normal, even when I'm driving and fiddle with the gas pedal, like between the gas and I hit a certain rpm, I'll put that 4 in there.
When I'm stationary it goes normally