no fourth gear in mitsubishi gto automatic

Could be a stuck shifting solenoid/valve body-get it pressure tested by a transmission specialty shop

Check fluid level, condition, and pressure.

the fluid level is fine, the overdrive is now a neutral, could the gearbox be gone??

Have you had it pressure tested?

What is a pressure test thanks

A pressure test determines whether or not the transmission pump is working properly and supplying enough pressure. One of the tranny cooler lines needs to be disconnected and the pressure gauge is installed.