@Akiko...If it does not support the Mediator, then it kind of puts a halt to the idea of installing it.
Is there a guid anywhere that details how to set up a dual boot for OS4, OS3.9, and maybe MorphOS?
It's pretty simple. You just need 3 bootable partitions (probably best to give 3.9 the highest priority), then select the one you want from the Early Startup Menu. You also need to force a bootdevice in OS4's Kicklayout and MorphOS's startup-sequence. Otherwise, they'll try to boot from your high-priority/3.9 partition, which will cause all sorts of problems, and then probably crash.
If you don't want to mess with forcing a boot device, you'll need to change the boot priorities in HDToolbox every time you want to use a different system. Rather annoying, but serviceable.