I have flown Air Asia plenty of times although never on that particular route. They have the pros and cons of any LCC, i.e. mostly reliable, but in case of a cancellation or schedule change you will most likely not be put on a different carrier. Personally I would probably not be comfortable with just a 4 hour layover, but it depends on when the flight is (i.e. the likelyhood of a schedule change), and what the consequences are if you arrive late in BKK.