I had a Mk IV, and now own a DC-5 which I like better. Tonally, the Mk IV can sound like DC-5, but not the reverse, as the Mk just has too many options for sculpting the tone, but that's where it ends. In my opinion, the DC-5 has a better clean channel, better reverb, is easier to dial in (although I've never had a problem dialing in a Mk series amp, either), has more gain on tap for the lead channel, and the lead channel is less compressed sounding at volume. The DC-5s lead channel also doesn't sound as "dated" to my ears as the Mk IV's.
With the Mk IV, due to the shared controls for ch 1 and 2, you're always in a struggle for a good clean on ch1 vs an okay (at best, IMO) crunch on ch2. Whereas with the DC-5 I can turn the gain up on ch1 and ride my guitars volume control for either warm, round, sparkly clean with the guitars volume rolled down a shade (with a better reverb that smokes the Mk IV all kinds of ways), or turn it up for AC/DC-James Gang type crunchiness. Personally, I never liked the sound of the Mk IV's ch1 turned up, and didn't like the sound of Ch2's gain turned down.
And as much as I like the DC-5's ch2, it does have a certain amount of gain and low-mids that simply cannot be dialed out. Back the gain all the way down to "3" and your still no closer to the sound of ch1 on "10" than you are to have it on "7" etc... If you like your amp's channel switching to sound like a logical progression in gain from one channel to the next, the DC is not for you. Tube substitution can go a long way to help in that regard, however.
I'm sure someone will be along to discount my opinions, but they are just my opinions, after all. I did use the Mk IV for three years as my stage and recording amp, and also used it to record a bunch of other bands who might not have had the best equipment and managed to have all of them sound like they were playing something different. I'm not so sure I would have been able to pull that off quite so easily with my DC-5.
It all comes down to the Mk IV is way more versatile, but I like the sound of the DC-5 better.