It's most certainly possible to have [incr Tcl] working with the Tcl 8.4 series.
bash$ tclsh8.4
% puts [info patchlevel]
8.4.7
% package require Itcl
3.3
% puts $itcl::patchLevel
3.3b1
You might prefer a slightly different version, but that combination works on my system (OSX Leopard).