I think that the best exp/hour-ratio comes in form of mp1. But still, with the recent buffs of the exp bonuses for higher monster powers it will be less of a problem to run different mp's. In the end, it depends on your playstyle.
Moldran ran spreadsheets on every MP on PTR on his GODLY barb. MP1 was THE fastest xp:hour ratio. If anyone here out gears him, by all means, go for a higher MP level... But hes probably one of the best planned out Barb's in the game as far as eDPS.
Well I don't know this guy and I certainly can't outgear anyone, but when mobs die in one hit on mp4, I don't see why I should bother with lower mp.
You won't one hit elites on MP4. That's where the time sink comes in. The time it takes to kill elites on MP4 compared to MP1 is VERYYYY noticeable. Do the math on 10-15 elite packs (im lowballing it here) in an Alkazier run. Let's say they take 5 more seconds per pack on MP4 than on MP1. Thats an added 50-75sec per run. That really starts to add up at the end of the day when you're looking for the best xp:hour ratio.
To build on this - if we know MP1 is the sweet spot I know I won't need to have 180+k dps on my barb, I can drop him to a comfortable 120k and wear full IK set, maybe a few pieces with MF that arent BIS but get me to my cap since im not paragon 100 yet, and I'm still blowing through elites nearly one shotting them - keeping my fury up all the time and getting more sprint uptime. Win/Win/Win