Get the Tk3 to Tk4 porting notes from
http://www.scriptics.com/doc/tk4.0.ps. This is also covered in an
appendix in the 2nd edition of Welch`s Tcl/Tk book. From Tk4 to Tk8,
there were fewer incompatibilities but more major features introduced.
See http://www.scriptics.com/download/8.0.html for a list of the new
features and some notes on upgrading.