How do you get back the use of the forth gear after it does not kick in any more?

Automatic or manual transmission?

sorry auto, it goes through the gears alright until it gets to the third it does not kick over to the forth as per usual

How's the transmission fluid level and condition/ If okay Possibly a shifting solenoid hanging up/valve body or get it pressure tested to find out more what's going on-

ok if i did a fluid change it would help if it is fluid problem if not should get it pressure tested. or should i get it test first then change fluid?

wat is an average cost to test?

Dunno the cost of repairs-if the fluid level and condition are good-have it pressure tested

What transmission model do you have?

Is the OD OFF light working?