OK, the dawn is breaking…
I should have caught this sooner: The option to "install OS9 Drivers" when formatting a drive was removed as of Lion and is not available in El Crapitan. So stuff placed there in your Pro is invisible to OS 9 in the G4.
Here is a simple, straightforward solution for all of this that has worked for me for years - not the only solution, but a good one:
Install Tiger or Leopard on the G4 - either on a separate drive or on a separate partition / volume. Make certain "Install OS9 drivers is checked.
Download and install TenFourFox as your OSX browser.
Install OS9 on the G4 - either on a separate drive or on a separate partition / volume.
Download and install Classilla as your OS9 browser.
Now, you'll be able to access anything, anywhere on the net with the G4.
AND
Now, you will be able to transfer anything you like between computers all day long.
You can transfer between 10.11 and 10.4 or 10.5 over ethernet - the computers will show up as available shared servers to each other.
If you have something on the pro, you send it to the 10.4 drive - say, the desktop.
Then you reboot the G4 into OS9, find the file on the 10.4 drive desktop and copy it to the OS9 drive.
You can even transfer stuff over Firewire with one of them in Target Disk mode.
Sooner or later you'll even get SwitchRes happening so you can see what you're doing…