In my experience, and it's based on a Mark IVa, you'll do well to go pentode, full-power, and possibly even simul-class, with pentode being the most important factor. I've had good luck with both EL-34s and 6L6s. From there, I like:
R2 Gain: 7-10 pulled (or less depending on master volume level)
Bass: 2-3 (turn it up until right before you hear low notes flubbing out)
Mid: 10
R2 Treble: 7
R2 Presence: 0 pulled
EQ: Top line, 1/2 between top and middle lines, middle, middle, middle
I roll my Les Paul's volume back to 8-9 and crank up the master as far as possible. Adjust the graphic EQ to suit your cabinet.
Personally, I've found that the Mark IV has a very different vibe than a Plexi style amp, and I've never gotten mine anywhere close to my Marshall, but you can get a great Mark IV rhythm sound with a lot of punch by going pentode, full-power, and simul-class. In fact, one of the reasons I love my Mark IV is that it doesn't sound just like a Marshall (although I love those too).