Can someone help me figure out why only one wheel is spinning?
Firstly, the 9V batery is going to struggle with driving two motors. Change this 9V battery out with another power source.
As @timemage points out in his comment, you are using a PWM command on a non-PWM pin. For the Arduino Uno, the PWM pins are: 3, 5, 6, 9, 10, 11. Move the wiring and change the code so that it is using pin 9, 10 or 11 instead of pin 4.
It sounds like you are trying to test your hardware setup and your firmware at the same time. It is much easier to test your hardware first and make sure that works as expected before testing your firmware. I suggest writing a simpler program (the simpler the better) that just drives the motors full forward - one at a time and then together. Verify that it works and then change your test program so the motors spin in (full) reverse. Then change your test program so it changes the speed of the motors. Then with known working hardware test your firmware.