From my research, the mark 3 has a slightly more brutal distortion, but the 4 has seperate channel eq's for the clean and distorted channels, so if all you care about is high(er) gain, save some money and get the 3, but if you're going to use the amp live and need pretty cleans, get the 4.
I really want the cleans and I play more alt-metal so I dont need crushing distortion. The Mark IV sounds great for what I play and if the cleans are better then I think I will go for that. Just the problem of finding one, and at a reasonable price.
Overall the MKIV is more versatile, with a better R2 channel and smoother lead. The MkIII has more aggressive gain overall. Either one is excellent, the MkIV will run about $300 more.

For a MkIV plan to spend around $1200, thats the going rate.
It's a fine line between clever and stupid.
The Mk III doesn't have as good of a clean channel as the Mk IV, so that could possibly be one reason to go with the MkIV.
Quote by davedoom
Mk4 Never Fails!

My Rig:
PRS SE custom
cube 60
Dual Rectifier Roadster (2X12 combo)
DR strings
EHX Multiplexer
The only problem I see with the mark 4 (I've been really interested in them lately) is how complicated it is to set them up. I was at a music store the other day trying one out for the first time, I couldn't even get an acceptable sound out of it, much less a good one (it sounded like a fart). Now, I honestly believe they are capable of awesome sounds, but neither I, my friend, or either of the two workers I got to help me could achieve them since the amp was new to all of us. My friend mark has the notorious mark 2c+, so I'm going to get him to show me how he sets his to give me a ball park idea of how to set the 4 for next time.

Also, the threadstarter should look into a used f50 if they want something a little more simple, we tried those out the other day, my friend had it set for excellent tone in a matter of seconds, and while I believe the marks are capable of better tones, I'm considering going with one of those for convenience...
if you are still considering a Mark III, make sure it has the graphic EQ if you are looking for metal tones out of it. As for getting good sounds out of it, the easiest way is to use the factory suggested settings in the manual, and then tweak it to your taste. It's easy to get a good sound out of it that way IMO. I've found it easier to dial in then a recto tbh.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
amp clips
amp vids
MkIV's have a lot of different tonal controls, you need to take the time to read the manual to understand how they all interact. Once you understand how it works you can dial in a ton of great tones.

20min at a guitar store isn't the way to go. Best bet if you don't have a manual is to hit up the Mesa forums for settings.
It's a fine line between clever and stupid.
Sounds like it would be best to get the Mark IV, at least for my needs. Does anyone know where to get a used one besides ebay?
Ive been looking on musicgoround.com, do you know how often, if ever, they get Mark IV's in.
I heard that MIVs are more Marshally.......
Tone is all ...... well probably 75%, in your fingers.
The rest depends on your wallet's thickness !!

Keep the faith, baby!!