125-200cc Wr 250 Wont Shift To 4th Gear

Yesterday I got a new bike . Its a 1999 wr250 . When i took it for a ride i could only shift up to the 3rd gear . So what should I check or replace?

I would put it on a stand to get the rear wheel free
then try shifting through the gears while running the wheel front and back
it will be more relaxed and can tell if the shifter feels bound like a bent fork

Shifter feels like if im in the highest gear , like there is no other gears

very bent shift fork

yes it's a bent fork
what happens is the shift drum loads up against the fork and if it won't move the drum won't rotate, hence the shifter feels like it hit a wall

Shifting forks from cr250 will be good? or from wr125?

So When the bike was on the Stand and tried shifting and the problém is that the shifter doesnt return to the originál position . Its 1-2 cm higher or lower . When I return the shifter i can shift to another gear . Can it happened from just sitting ? Because the last owner rode it just 50km in the last 4 years .

are you saying that on the stand and some monkeying around you got past 3rd gear??
since the bike is not in good service I see 2 choices, park it or gut it like a fish
I would pull the clutch cover and take a hard look at the shifter parts if it did shift when you finagled it, besides that comes off anyway if going deeper
I wouldn't put my hopes too high but mechanical things can surprise us from time to time

As said .... remove the bolt to the shifter completely.. remove the shifter.... then remove the clutch cover and have a look see....

Then with the clutch cover off ... reattach your shifter.... put it in first gear .. with your hand on the shifter push it down...
remove your hand .... let us know if it springs up

So I changed the oil in the transmission and there was mixed oil with water , so im in the middle of flushing the transmission and it seems better , shifter returns most of the time

you might need to change it more than once if it's so nasty to stop the shifter from free movement

For a flush I’ve used automatic transmission fluid ... can you go thru all the gears now.

yes i can go thru all the gears

Good to hear ... flush it a couple times with the ATF fluid .... 800ml of oil .. this is what I use to flush mine ...

Usually a ride or two then drain