It depends what do you mean by "default" browser.
If you just want to get to one version by typing its name on the command line, put its directory first in your path.
Ex:
/usr/dt/bin/netscape is where netscape 4.0 is
/opt/netscape/nestcape is where netscape 7.0 is..
if your $PATH is:
PATH=/opt/netscape:/usr/dt/bin
iwhen you type netscape on the command line, netscape 7 will launch.
If you want to click on the "world clock" menu icon and lauch netscape 7.0, then you'd have to create what's called an "action", which is just like a shortcut, and put it in your menu bar.
If you want to use the home folder GUI and double click on an html file and open that file with a specific netscape version, then you better get the CDE FAQ (or Gnome, KDE, etc) because I have never actually use the home folder GUI. :)