I've encountered this problem on systems where the startup disk values in PRAM/Open Firmware are corrupted.
Try each of these key combinations (hold during startup), and wait for the system to reboot twice:
Open-Apple + Option + P + R
and
Open-Apple + Option + N + V
You can't boot the G4 directly from another system, however, you can do one of two things:
Start the G4 in Target Disc Mode:
Connect the G4 to the G3 with Firewire. (If the G3 lacks Firewire, this is not possible.)
Hold the T key when starting up the G4.
A firewire icon should appear on the screen.
The internal HDD of the G4 should mount on the other system, to which it is connected via Firewire.
Create a Netboot or NetInstall image on the G3, and connect the two systems via Ethernet.
Note: I have to disagree with the modification, purely because the OP is entitled to this software, per the Apple license that came with his system.
From Apple's List:
iMac (USB 2.0) 17-inch 1.25 GHz, 15-inch 1 GHz * Aug 2003 9.2.2, 10.2.7 10.3.1 6R42, 6R49, 6R52, 7C150
The HW license affords him the right to OSX 10.3, which is what I provided in the form of an emergency boot CD.
Apple no longer provides support for this system, and the software is not easily found. A boot CD that can only be used for the purpose of recovering system datum is hardly a serious breach of any kind.