Yes, they're quite different. The name & speaker size is about all that's the same.
Well, and power - with the 8-ohm speaker I got, the Mark IV is allegedly putting out about the same as the Max 250 does with no add-on cab.
The Mark IV: it takes a bit more boost to the bottom end to where I want it, but easier to bring down the highs. If that makes sense.
The Max 250: super easy to get the bottom end up, even without the Kosmos-C. Also easy to make too boom-y. But I've found it difficult to get rid of a high-end "twang", especially on the G-string.
A few minutes of fiddling with knobs & switches & such that I don't understand fully, yet, on the Mark IV, and no twang, no boom, sounds great
And a voice modulator & dark lighting?

Something like what they used to do on news shows, when they didn't want to reveal who the "witness" was?