Forum OpenACS Q&A: Response to OpenACS 3.x or 4.x by year end?
In my opinion use 3.x if:
<li>Site has many users and requires good performance. Since 4.x permissions may not yet scale well.
<li>You need a module has is not yet present in 4.x and is very hard to port or no 4.x modules may substitute for it.
<li>You are sure that the site will not use 4.x features now and the future.
<li>There is little chance of upgrading to 4.x
Use 4.x if:
<li>site needs or greatly benefits from the new features of 4.x like complex permissions, CMS/CR, templating, etc.
<li>site may need or outgrow what 3.x offers.
You may also want to talk to your client regarding this. Well you have to put into lay man terms the problems and scenarios. For me I picked 4.x on site since it was relatively light load and deadlines can be moved. I picked 3.x on another site since I need to make it in 1 week and did not require any 4.x specific features. Look at the short term plus and minus if its going to be a short term project otherwise look at the long term plus and minus. OpenACS 4.x is not in a bad shape, infact its in a good shape considering its on alpha release. Also bear in mind that OpenACS 4.x was based on ACS 4.2. Good luck in your work.