I do have an EP4000 and an iNuke3000, but have no experience with the EPX.
However, before I bought the iNuke, I had perused the EP and EPX specs. and made a bit of a comparo between the EP4000 and the EPX4000.
The EPX4000 has a switched mode PSU vs the EP's heavy iron.
The EPX has a higher input sensitivity (and a bit more gain), weighs 14 lbs. less and is an inch shallower vs the EP4000.
I like my EP, but its idle heat output is oppressive. The iNuke is frigid by comparison (plus, the iNuke is a flyweight and it's tiny).