Forum OpenACS Q&A: Response to OpenACS 3.x or 4.x by year end?
BTW - my comment about ACS 4 compatibility only applies to the *Oracle* version of OpenACS4. This guy's using Postgres so there's no avoiding the porting issue.
OK - back on topic. The strongest argument in favor of OpenACS 4 vs. 3 would be the Content Management tools. For your use, Musea's "Edit-this-page" may well suffice. It's relatively easy to build instances that look a lot like the traditional FAQ and News packages as well as more vanilla article scheduling/processing stuff.
The strongest argument against is that not all packages available in the ACS 3 world are available in the [Open]ACS 4 world. aD never completed porting stuff over. We'll be seeing packages with similar functionality appearing over the next few months but will it be in time to help you?