I have a Mac G4 with a corrupted operating system. I don't want to reload OS X again, but want to try Ubuntu on it. If I download the current release of Ubuntu on a Windows machine and burn the ISO, will that load on the Mac?
PowerPC is not officially supported by Ubuntu, but you can download a PPC version here.
You can download and burn Ubuntu from any computer, and the resulting disk will load and install fine on more-or-less any computer that has a CD or DVD drive.
However. By G4 do you mean the line of Macs that was discontinued in 2004? Those computers use PowerPC processers, which are now no longer supported even by Apple. No supported release of Ubuntu will work on it. Unfortunately, your computer may be the "less" in "more-or-less any computer" I mentioned above...