York Diamond 80 Furnace Blowing Cold Air. About five years old. Red light is flashing four times.

4 flashes = High Aux and spill limit switch open. What are some suggestions to correct this?

Try to reset if possible. Many control/temperature disks have a small red reset button in center. If not test controls and replace if defective. The next step will require some HVAC knowledge from a on site service person. Locate the reason for the control that has been activated. In other words they do not trip without reason. Closed registers,dirty air filters,blocked return airs, clogged discharge flue pipe or other items could be a problem and that is what tripped the spill or limit control to start with.
Good hunting.

I found five control switches and all of them has small reset buttons but none of them seemed to click. I did have some closed registers so I opened them, I checked the furnace's internal filter but it wasn't clogged and the intake filter (in the hall) is almost new. I also took a look down the discharge flue and didn't see anything blocking it.

I installed this unit (with AC) myself. While I am not a licensed contractor, I did remodel most of my home on my own using books and asking a lot of questions. Everything that has been inspected has passed, including my HVAC (I did get a contractor to finish the brazing and charge the system). I realize that times are tough and contractors need work but I am currently between gigs myself and need to keep my family warm so I'm hoping I can get a little more guidance than "call an expert". In any case, I appreciate whatever help you can offer.

Many control/temperature disks have a small red reset button in center. If not test controls and replace if defective

Did you test the controls with a meter?