Actually the newer models have an improved and cooler chipset in them so if the newer models get the RRoD its not due to it overheating.
3 Red lights means its an internal hardware failure, it could be a number of things, overheating being one of them but thats not always the cause, I've found with alot of people that when their 360 gets 3 red lights its due to it not being ventilated properly, if it has decent airflow and is ventilated then I don't know what the reason could be.
4 Red lights means that your cables aren't connected properly, some people think 4 lights is worse but its not.