According to this article it appears there will be a good sized audience for OpenACS 4. There are some people out there still looking for the "killer web app"

http://slashdot.org/article.pl?sid=01/08/14/2327248

The article claims people are looking for more than just simple web serving.
They are looking for web applications.

OpenACS 4 provides a skeleton on which to place web applications and
comes with several fine applications out of the box.  (cms and e-commerce
being obvious ones).  I expect to see more web applications produced for
OpenACS 4 after it's release.  That puts it in a position to compete with the
other web application type platforms with the advantage that you don't get
nickel and dimed to death ($50 for file upload here, $300 for an e-commerce
module there) and that the modules are already designed to work together.