Aha, according to the instructions I used last time when it worked, plpgsql is loaded when you load it. That is do this:
createdb openacs-4
createlang plpgsql openacs-4
But it would be nice if the installer could check and set it up for you, or to make it part of the basic datamodel install.
Check out OpenACS 4 installation.[1]
[1] http://openacs.org:7000/file-storage/download/openacs-4.html?version_id=49