Hi Rick,
1. I believe OpenACS 4 is stable enough we have rolled out production sites with it. Some sites are heavily hit sites, although we are growing to about 10K acs_objects a month. We do purge some of them.
2. You may need to break some old habits from OpenACS 3.x or ACS 3.x, this is what I have learned. Just read the docs and approach 4.x with an open mind.
3. I use source since it gives me some custom capabilities. If you are now familiar with OpenACS I suggest that you try to use from source. See it you need the extra customization if not continue to use RPMS.
4. Just read the docs and know why was 4.x designed that way. The object like way of using the RDBMS has advantages and disadvanteges, as well as the concept of packages and mounting application. Just give the 4.x style of framework after a while you will get to used to it and appreciate it. Dont go againts it or it would be better of just using 3.x and extending it.
