Baron, welcome. Your first question can't be answered with any more precision than a "maybe". An amplifier/receiver doesn't know the impedance of a speaker to which it's connected; what its protective circuits do "know" is whether current flow and/or operating temperature are too high. In either case it'll shut down the unit to prevent damage. There're many factors which can affect this, e.g., size of room, distance of listener, dynamic range of material being played, average listening level being used, sensitivity of speakers, etc. The M80s are slightly above-average in sensitivity, have a flat impedance curve, and have been said to be an "easy 4 ohm load". A published example of using an M80 setup successfully with a similar or even slightly less powerful receiver than your 898 is Clint's review of the 85 watt Onkyo 601 at Audioholics
. So, the fact that a unit isn't lab tested under the severe standards required by the FTC and UL for an official 4 ohm rating doesn't necessarily mean that it wouldn't work well with 4 ohm speakers in actual home use.
Sure, the M80s can play more loudly with low distortion than the M50s and have a slightly different timbre. If you're concerned about the 4 ohm thing, note that the M60s are closer overall to the M80s.