I've only played a IV once, and it didn't suit me at all, but I didn't get to play it in a band setting
If my bandmate had one or the other it might be a different story...The recorded sounds of the IV seem to be supreme on every album/recording...even on netmusicians!
To me the IV sounded "boxier" and way more midrangey, but yet again, I found out Vic (phyrexia) had it accidentally hooked up in the 4ohm slot versus 8, and I was totally shredding my brains out; about 15 minuts later, it blew one of the outer EL34s. Secondly, we had it waaay loud and it was loaded with all Groove tubes; not my preference, I like russian tubes (EH/Mesa 440s, etc)...could have explain the bland tone and difficulty dialing in "my tone".
Lately my Mark III has started having that "boxy" tone as my crappy Ruby tubes have started trying to give out and I end up having to drive my lead master higher for more saturation in the band mix; it's also losing gain. I need a new solid v2 and power tubes. My R2 sounds hissy...so...
In short (lol rather in long)...it's a topic that gets discussed alot...the choice is really yours, but I've personally been itching for a new Mark for recording with. The III sounds amazing live, but as i've discovered takes some "massaging" to sound more like one or the other on record. Mainly on the cabinet or microphone end, as there are very fine nuances you will get bothered by saying "I didn't hear that before" or "Those frequencies weren't there earlier!". Otherwise the Mark III dominates on the lead channel.
Theres my $0.04
(edit: oh and apparently the Mark IV is supposed to be sensitive than the III but i didn't notice any more sensitivity to the dynamics of my pick attacks or articulation. Both are equally badass; not to exclude that a) I blew up the one Mark IV i've tested in under an hour, but b) i've used all sorts of amazing tubes in my III and had it as perfectly close to a IIC+ as I think I can get it tone wise without switching to the 105PT, imho)....many are sure to argue here.