@Nearly-Right
I am sorry to hear you had difficulties. OS4 classic should work "out of the box" with the Mediator and Voodoo. There are not extra drivers required. You do have to pay attention to the installation which, I believe, asks you if you have a Mediator or not.
Unfortunately a bug was discovered after release with the "LT" version of the Mediator - which is for A1200's. A fix is coming soon, so hang in there. OS4 classic is definately worth the wait and will breath new life into your system.