Forum OpenACS Q&A: Response to OpenACS 3.x or 4.x by year end?
How much do you value the ability to *easily* add new modules to the site that have been created by third parties? Do you have lots of content that needs the new version 4 permissions? Do you need the new version 4 templates? How accepting of risk are you? Very? Moderate? If yes to many of these questions, your way is clear my son.
If no, then you may be happy with Version 3 which as you say, has been tested by time....
By the way, I haven't tried it, but I believe Don says that OACS 4 should support at some point ACS 4 modules. If so, then there is an adserver for 4. I have a version of it for ACS 4.0.1 that folks tell me broke for some reason in 4.2. I dunno. Furfly probably has a fresher version.