J-HFS is fine,
on the G4, yes, turn on personal file sharing in the sharing preferences (see attached image). note the computer's name in the top (use a dash for spaces. after the two are connected via ethernet, from the G5 finder, hit command-K, type either "G4 computer's name.local".
connect to it using a the G4's admin's name and password
you should have read/write on almost everything. If not, since you're logged in to it as an admin, you can change permissions if you need to.
edit: Obviously the G4 will be headless. I like to run chicken of the VNC to control it remotely (i.e. use it as a fully functional computer, rendering etc. at the same time as a file server)