opendisplay - https://github.com/peetzweg/opendisplay - can make a old iMac's screen useful again. In my case, a 2015 27" iMac can be used as a monitor for my modern Mac laptop, connected via ethernet cable.

Very cool, I like this a lot. Will be giving it a try!

I've had good success with https://juicycrumb.com/product/docklite/#imac-model

There is a g4 model too there if thats the version you have.