Hi it means there is an open priXXXXX XXXXXmit. I suggest you turn the power off, replace any air filters you may have turn it back on and it should restart.
I replaced the filter, the blower and exhaust fan run, the gas valve does not click and the indicator light is still flashing four times
Right well its off on a limit look around the burner area for some small usually black plastic and metal things with wires attached to it usually 2 wires and there should be a small red button in the center. there can be a few of these but if you reset it it might work, but they usually trip for a reason it is rare that they fail but it can happen you'll need someone on site to verify this condition as these safeties are there to protect you.
I checked the limit switch, but the button is not up--I tried to push on it nothing happened-
could it be an airflow problem with the return?
well if its off on limit that means the furnace is hot usually. You could have a bad limit. What is the complete model number?
model- guco 90c 35c
Air problem or bad limit thats all we can do. Hope it helps. :)