I have a LTD snakebyte with active Hetset pickups. I play mostly rhythm so I want that 80s Metallica thrash rhythm tone. I also have a MXR 10 band eq pedal if need be. Right now I have a Peavey 6505+ head with a marshall cab and have a good tone.

Is this a good price for the Mark IV and do Mesa heads work well with Marshall cabs, can I get a real good rhythm tone out of the IV?
That's really up to you!

Mesa heads work perfectly fine with Marshall cabs. Mesa markets their Mark IV and V as "the last amp you'll ever need to buy." To a point, that's true. If you're looking for the quintessential 5105, or JCM800, or Soldano sound, this won't be it.

If you're looking for something that's got a character of it's own and can get you 80% of the way to sounding like every amp out there, then yeah, that's kind of what the Mark series is aimed for.

The Mark IV can handle that 80's thrash sound very well. In fact, many amps can. The tone for that genre isn't really super defined to one specific sound, so there's a lot of play. Overall, though, you will get a really saturated, but still rather transparent distortion.

As far as price, I don't know. I don't shop Mesa so I couldn't tell you if that's a good used price.
I own one and I can make it djent. You just have to be patient and realize it has more sensitive pots. A working understanding of gain stages helps too, but once you know the beast you can master it.

I have a ns2 and ts9 and it can sound massive. definitely not a one trick pony.