Here's an easy thing to try, if you have not already simply moved the disk to a Mk2 for the diskbuilder step:

Install the Mk1 developer image, with the disk still in the Mk1. This will fail at some point, but ignore it. Then, reboot the player with the serial link hooked up, and hit 'q' or 'control^C' to quit back to a shell prompt.

From the shell prompt, try:

mkswap /dev/hda6
swapon /dev/hda6
mke2fs /dev/hda4 <--- Peter can tell us the exact parameters to use here

Then reboot and see what happens.

Cheers