Hi,
Can I suggest you start from scratch and get ONE Hall Effect Sensor working, before trying to get all 4?
Start with a blank code and build up just for ONE Hall device.
Forget for the moment about output control.
One you get code for one device working, then add another and get it working, then the third working and the fourth.
Use Serial Monitor to help debug.
Once you have the RPM working , then look at your algorithm for detecting RPM differences.
Only attempt the next stage after getting the previous working.
It will keep your bugs to a minimum.
Can you please post a copy of your circuit for ONE Hall device, in CAD or a picture of a hand drawn circuit in jpg, png?
Tom... 