These aren't the easiest to keep, four years is a great number, as far as what happened, only you would probably know that, too man variables to guess on this side. Personally I try to avoid fish that have been known not to do well in captivity, even when your the exception, Sorry for your loss, I'm sure you loved that fish, they are beautiful. Maybe SteveS would take a look for you & shed some insight.
WoW I see that lots of things changed, salt mix, removing sand, & did I get that right switching tanks?
I never seen large volume WC bother fish but changing salt mixes for no reason I had major problems, normally they recommend very slow & small changes for that. I did remove a DSB with success but I never had fish that sensitive either. EEL? Every eel I've see will eventually stress fish to say the least, like I said too many things going on at once again sorry for your loss, you did great for a long time with it, I take it you had plenty of coralline right?