Nothing has been confirmed about the mark IV being discontinued.
There was RUMOURS of a mark V, but also rumours of a triaxis v3. But we haven't heard anything insofar.
As for cabs, firstly
A recto cab is the obvious first stop.
As for me, I couldn't justify the price so I got a custom avatar cab and its working great! I have a 2x12, with a vintage 30 and a g12h30 in it and it sounds sweet with my mark IV.
www.avatarspeakers.com