Dan, at this point being even "a little concerned" would be premature. None of the receivers which you might consider carry an "official" 4 ohm rating. To do so they have to comply with FTC regulations which require, among other things, that they would supply the full rated power into 4 ohms for at least five continuous minutes. None could pass this laboratory test, which would have no significance for real-world listening.
Your 6080 or similar receivers(if you get a different one because of better features, not an imagined 4 ohm problem)should power M80s with no difficulty to all safe listening levels.
As was said, the 4 ohm impedance setting should never be used. It limits output voltage(and consequently current and power)for the purpose of reducing the risk of overheating. Tests show that this cuts the maximum power available roughly in half, so this cure is worse than the disease.
