Thanks brsaw and tomtuttle!
I wanted to give everyone an update. I have successfully transitioned from my two EP400v3 subs to my new EP500v4s. I was happy before the upgrade, and I'm still very happy now.
First off, I have to say that after doing the upgrade, I have all the more appreciation for the output that the EP400s are capable of. Anyone who requires a smaller sub due to space considerations would would be very happy with these
As you might guess, the EP500v4s are capable of higher output without having to "work" as hard, as evidenced by less dramatic cone movement. Subjectively, I am fairly certain that I am hearing less compression, and deeper lows at louder playback levels with the 500s as well (for various possible reasons). To be fair, this was also a v3 compared to a v4 and so the new V4 amps may also be contributing to some of the differences I am hearing.
If you are trying to decide whether to go with the 400 vs the 500, I can tell you honestly from my own experience - having used the 400 regularly for critical listening for over a year - that the 400 is a fantastic sub that sounds much bigger and deeper than you would expect an 8" sub to sound. Whereas you could go with the 500 if you have the room for it, and require maximum loudness.