It seems to boot the amiga kernel, but the kernel isn't able to find the root on the CD.
What happend connecting an USB floppy reader and a boot disk?
I suppose that the floppy must be in High Density (my usb floppy reader for example don't read lowdensity format)
How did you find "slb"? is a file on the OS4 cdrom? Does someone else files is present?