One option for having a partition accessible to all your OSs is to create a FAT partition. This is readable by Linux and AmigaOS (using FAT 95) and probably any most other OSs too. The FAT partition can small and be used purely for file transfer.
Obviously I can't say for sure it would work with OS4 but this solution works very well for me with Amithlon on a multi-OS machine.
HTH
Robert
