OSX iMac G4 install: possible from USB?
This won't be possible, for a few reasons...


Snow Leopard was the first Mac OS to require an Intel processor.  This is stated as the first requirement on Apple's Snow Leopard Technical Specifications page.
The vast majority of PowerPC Macs did not support booting from USB devices.  Towards the end of then PowerPC era Apple rolled out a change to Open Firmware in the last models to enable USB booting.  Your G4 iMac though would not be one of these models.


You're left with two options...


Create an admin account.  This can be done by removing the .AppleSetupDone file, forcing the Mac to go back through the initial setup, part of which includes creating an admin account.  The following steps should walk you through that process:


Shutdown as normal
Power on your Mac while holding command+S to enter Single User mode
Type rm /var/db/.AppleSetupDone and press return
Type reboot and press return
The Mac should now reboot and go through the setup assistant, allowing you to create a new admin account.

Or, reinstall the OS, like you're wanting to do in your original question.  If you can find a copy of Tiger or Leopard you will likely be able to install it using the optical media (CD / DVD).  If you can find an image of a Tiger or Leopard OS / Installer then you can install it by using a Firewire 400 equipped hard drive.


Hope that helps.