I have a comfortmaker central heater unit that won't stay on--there is a green blinking light on top of the gas regulator--what does it mean?

They should be explained somewhere on one of the two front panels.

The LED bliking light gives you error codes as NortherHeat stated. The bright / dim type flashing is known at the heart beat, no problem. If the light goes completely off and on, it is blinking an error code, followed by a pause, and it repeats itself. Count the flashes and refer to the error chart on the inside door of your furnace... To clear this error code, shut of the power to the unit for about 15 seconds and then repower up the unit.

The green light is flashing 4 times and saying limit switch is open what does that mean?

That means the high temp. safety has opened to shut down the burners. Most likely cause, is a dirty air filter, closed feed and/ or return vents, as well, too low of a blower speed on the circulation blower. Let the unit cool down, should rest the main limit. For sure the cause needs to be found to insure a safe operation of your furnace. If you have further problems, post back your issues...