Car not going to 4th gear?

We need to start with checking the fluid level and condition. This is going to tell us what the transmission is doing because if the fluid is dark and smells bad then we have a clutch slipping or something causing excessive heat.

https://www.2carpros.com/articles/how-to-check-and-add-transmission-fluid

Once we check the fluid, we will need to check the torque converter solenoid and make sure it is operating.

Here is a test that will help with this:

Please run through this and check the resistance of the solenoid.

Here is a guide that will help with this:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Let us know what you find. Thanks