Mark, In terms of support by openacs.org I would say there 
are a number of us who have been using 4.0 regularly through the 
beta period so there is plenty of expertise about how to 
get it to work (and I have not encountered any problems with 
it since beta10 I think).
I believe internally AOL has cut over to 4.0 (digitalcity.com
definitely has, not sure about other sites) so I am pretty 
confident it has gotten a good workout and is production ready.
Yes, I think we should switch to 4.0 as the recommended 
version although I am not sure we can get the install docs
revised in time for 5.0 so it might happen with 5.1.