I was wondering if there are any unimplemented (but planned) features or
known bugs for BackupPC 4.0.0 (thinking of general stuff needing fixing
before releasing 4.0.0).

I'm not aware of any current bugs, but that demonstrates the need for more testing rather than readiness for a production release.

There are a few features I'd like to add for 4.0.0:
 - parallelize the V4 pool reference count updates and scan, which is run each night (I just implemented this).
 - support FTP transfers (BTW, does anyone use FTP?)

There are some more significant features that I'm starting to think about.  I'm not sure any will make 4.0.0 because some involve significant changes:
 - rewrite the main scheduler to allow host groups (an often requested feature) and perhaps the ability to do absolute scheduling of backups (ie: do a full every Friday night at 11pm).
 - add plots using rrdtool based on earlier work by Ludovic Drolez and Nicholas Hall, with recent improvements from Alexander Moisseev.
 - look at SCGI support, allowing the BackupPC web server to be decoupled from apache.  That would avoid any setuid scripts or the need to run apache as the BackupPC user.