Will not shift in to fourth gear

This sounds like it can be one of two problems first how are the shifts from first to second? If it shifts late then you can have a plugged catalytic converter. Here is a guide to confirm the issue:

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

If that checks out it could be a input shaft speed sensor that is not reading correctly. Here is a video of a sensor getting replaced on another car and the location of the sensor for your car is in the diagrams below:

https://youtu.be/f3bETtCIdxM

Check out the diagrams (below). Please let us know what happens.

Cheers, Ken