M-4 refers to the sizing. I've never seen anything higher than 4 (though that doesn't mean it doesn't exist. I'm only going off of what I've seen).
It goes: S 1-4, M 1-4, L 1-4, and I think there's an LX 1-4 if I recall correctly. I fit fairly comfortably into an M-2.
The protection level is the same across the board. The newer ones seem to be a tad more durable, though I'm not sure how important that is if you're not in combat. They're better at fragmentation than anything else.
Most used kevlars go for about $50 give or take a few bucks.
There's some great padding you can get for them, though it costs about $200. It makes them more shock resistant on your head and MUCH more comfortable. Makes it look more like a motorcycle helmet really.
I bought the pads even though I knew I was going to turn it in a couple of years later. It was still well worth it if you ask me. Those sweat bands suck so fricken much.