I like this idea, even though I don't have any toons that have ERed yet.
Nevertheless, if there is life after ER4 I will endeavor to get at least one toon all the way there (eventually)!
So, what is the concept of progression past ER4? I know toons that ER can change classes, feats, etc. during the ER process. Would they then progress with their ER4 as a baseline for the rest of their levels 31-40?