Apparently, QuarkXPress 6 will not be downwardly compatible to version 4. So, if I upgrade to 6, I have to rely on all the printers, bureaus, publication production depts, etc. that I use, upgrading as well. Otherwise - problems.
Quark's stupidity amazes me sometimes and with this one, I think they're playing right into Adobe's hands.
I'm going to have a serious look at InDesign.