This PDF is geared more towards the Perl/Tk programmer who is interested in the changes between pTk 800.025 (based on Tcl/Tk 8.0.x) and pTk 804.025 (based on Tcl/Tk 8.4.5). So look in the lower-right corner. Corrections, additions and the like are welcomed. This information is not meant to explain the new features, just to point them out.
When pTk started it was based on tTk 8.0 (actually Tk 4, but I'm not going there). As tTk evolved through the 8.3.x series, major changes were patched into the existing pTk code, so even though we'd say pTk is 8.0-based, it still incorporated the dash patch, image enhancements, Entry validate, etceteras. So the pTk 800.xxx series was pretty much Tcl/Tk 8.3.x (more (Tix) widgets perhaps, but minus two big features, Mac OS Classic and unicode support).
The pTk 804.xxx series is reasonably equivalent to Tcl/Tk 8.4.5, with the notable exception of missing Mac OS Aqua support.
Much of this data came from the wiki pages Jeff Hobbs provided. I also consulted the change logs from the Tcl/Tk and Perl/Tk source distributions. You'll notice that there is no 8.2 "column", which means that there were no significant Tk changes in that Tcl/Tk release.