I had to force quit; it then immediately started back up with 1.4b8. The only thing that may be related is that I had been logged out of Facebook chat and there was an authentication window in the background when I clicked 'Install and Relaunch'. Even after typing my password into that window, Adium was still stuck with the 'Install and Relaunch' window showing full progress. Otherwise it was still functional, I was able to send and receive messages.
Just FYI, 1.4b8 seems to be working fine.