Mike,
No clue why that warning is coming up. JPATH_ is definitely not defined.. it should be something like JPATH_SITE, JPATH_COMPONENT, JPATH_ADMINISTRATOR, etc.
Just searched our code for JPATH_ (with nothing else), and couldn't find it.. so a little stumped. We definitely installed on multiple sites and have different tests that didn't catch this. Hopefully, just chock it up to strangeness.
During installation, we update the JFBCLogin module and all the plugins. None of those changed with this release, so really, it shouldn't affect anything (if you were on 4.0.2) if something wasn't installed properly... just so you know.
Thanks for the report, and we'll keep an eye out.
Alex