Can't boot from USB or CD/DVD
I happen to have a 17" iMac 4,1. I burned the lubuntu-16.04.5-desktop-i386.iso image to a RW DVD using a 2011 iMac. I booted from the DVD using the 2011 iMac and ran the software on the DVD to verify the DVD was burned correctly. 


  To boot from the DVD, you need start the Mac and immediately hold down the option key until the Startup Manager icons appear. Select the DVD icon labelled Windows.


Booting from the DVD using the iMac 4,1 was problematic. Basically, at some point the boot would fail. At least once the boot succeeded. After some research I learned I needed to hit the F6, then include the nomodeset option before selecting Try Lubuntu without installing. Once booted to Lubuntu, I could select the install icon on the desktop to start the installation process.

My iMac 4,1 has the latest firmware updates which are listed below.

Boot ROM Version: IM41.0055.B08
SMC Version (system): 1.1f5


The firmware information can be found under "Hardware Overview" in the System Information application display. Firmware updates for legacy Intel Macs can be downloaded from the Apple website About EFI and SMC firmware updates for Intel-based Mac computers.

Additionally, my  iMac 4,1 has 2 GB of RAM installed and has a 1.83 GHz Core Duo (32 bit) processor.