Hey,
Been in the business 20 years, on my way out now cause of knee problems and just over it mainly. Ok quick rundown of things that cause a unit to freeze up. Main things are lack of airflow across the evaporator coil (indoor coil on the furnace or in the air handler in an electric heat case) caused by dirty filter, blower motor not running or running too slow, squirrel cage very dirty or loose/missing blades, dirty coil, too little duct work - either returns or supplies. Next is low freon - too much freon generally raises the pressure of the system and gets the suction pressure/temperature above freezing. Since unit has been running good for a period of time basically rule out a ductwork problem as it is mainly a design issue. Motor could be dying and running slow but most likely is low freon or dirty filter or coil - for the most part it takes a very dirty filter and pretty dirty coil to cause freezing if the system is correct and good in all other areas. I would thaw unit out good by running fan - on not auto at thermostat, and cool to off. You can keep an eye on the drain line outside to see once it quits for a little while you know the coil is completely defrosted - if very frozen this can take 6-12 hours. Start the a/c, if the inside unit is easy to get to go there, if not go out to the a/c outside, feel the suction line - bigger one, should be insulated but you can find a way to feel the pipe. See how cold it gets after a few minutes running - this will vary alot depending on the temperature inside and out, humidity ... which is why it is important a professional check it out - of course a true professional is not so easy to find. if it is an airflow problem you will feel the line get cold - especially at the inside unit. If it is low freon the line will not cool down quickly, especially outside - the cooling of the line comes from liquid freon still in the lines after passing through the evaporator so if it is low it will not let liquid through until after the coil freezes. How long does it take the coil to freeze after it is fully defrosted? with the unit being 4 years old is it an R-22 or R-410A system? Alot of the guys aren't up to speed on the 410A, it is much higher pressure than 22.
P.S. - you have changed the filter haven't you?
