Our iMac 4,1 (early '06 edition) was getting obviously long in the tooth, due more to the changes to the Internet and lack of further updates than anything else, so I thought it might be a nice project to try to install Debian to bring some more life into it.
I installed rEFInd without incident, and after a lot of adventures with making a suitable boot drive and such, managed to finish the installation using the 32-bit 3 gig build, with non-free firmware.
It boots up, to a point, whereupon the screen glitches out and it is apparently inert from that point on. A bit curious, considering that the graphical installer worked just fine.
Incidentally, during my research into figuring this out, I found a very similar set of symptoms that a Fedora user encountered back around 2011. That was a different version and a different machine, but the display problem looked so similar that I thought it was worth mentioning here for reference.