in norway where i live the prices on amp are pretty high. a new Mark IV is almost 5000us$, and a used one is half that if lucky i guess.
the mark III on the other hand is not that expensiv. can get one used for about $1300.
the sound i'm after is from pretty heavy metal downtuned to a more controlled distortion ala Dream Theater. The guitars played will be a Williams Axis with hot duncans (dunno exact what duncans tho), a Kramer Vanguard with 81/85 and a RG with evolutions.
will there be a big difference in the distortion between the III and the IV. is the IV worth the extra cash? i want a combo, and not a head+cabinet. and i wanna buy an amp witch will be loud enough and good enough that i don't need to worry about needing a new one in a few years.
(the amp i have now is just a cube 30W. sold my rig a few years back, but now i need something a bit bigger again)
If you search i answered this question but to help ya out

III- more agressive sounding, but lacks the a channel compred to the mark 4. think of a hot rodded marshall. pretty good amp

IV-smoother distortion, more versitile, more head room for clean channel

if you could get a III with a decent overdrive you're set.
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