I would think the end of the Mark IV is defintiely near. Let's face it, as previously said here, it's not the most popular amp in their inventory. The Rectifier changed the face of rock music in the 90's and is still going strong with variations after 16 years. Kids coming into their own these days playing everything from modern rock to death metal are still buying up Rectos by the truckload. However, the Mark IV has taken a backseat. It's probably been almost 10 years since I actually saw a Mark IV in a store, and when I asked about it, I was constantly told "We had one here for a couple years. It just collected dust and no one was ever interested in it, so we don't carry them anymore. We can order you one, though."
Everything changes over time. Just look at the Recto, released just a year or so after the Mark IV. Since it's release, it went through 5 revisions as a 2 channel, then it was discontinued to launch the 3 channel series. Then the Road King was made, and it was discontinued for an updated Road King II, along with a stripped down Roadster that was made. Along the way they also made rack versions in the Mesa Rectifier Preamp and 2:100 Power Amp. All of this happened in the same span of time that the Mark IV has been around, with sales paling in comparison to the Rectos.
With that said, I think it's time for an update. The Mark series was getting continuouly made over for a while - Mark I, Mark IIA-IIB-IIC-IIC+, Mark III Blue, Green, Red, Purple, and Black Dot/No Stripe revisions with Studio Pres, Quad preamps, etc...then finally they released the Mark IV and Triaxis...and they hit a brick wall, taking their focus completely off of them.
Where before, they were constantly putting R&D into the Mark design and "improving" it, it seems once the Recto hit the scene and became the new "it" amp, that's where their attention started to be focused. With it's massive success, it's no wonder that the Mark series has sat idle for pushing 2 decades.
But now with the Recto series still in full swing 16 years ater, and the Fender-ish Lonestar and Marshall-esque Stilettos picking up steam, I guess it was time to revisit old faithful and give it some much needed attention. Will they update it as a Mark V? Strip it down as a different variation? Who knows, but I have my doubts the Mark series would ever go completely away, and regardless of whether we think the Mark IV is good as is, there's no denying it's time for an update to maybe re-energize a series that has been long neglected.
I wouldn't worry about Mark IV prices going insane. Unlike the IIC+ which was produced for only a short time, the Mark IV will have been produced for about 17 years with more of them floating around than we can count. If anything, if it's replacement is any good, you'll likely see the prices come down on Mark IV's to reasonable levels.