4 or 5 GB is strange. Without OS 3.9 it should see 7.8 GB and with OS 3.9 it should be able to see the entire drive.
Nevertheless try this:
http://aminet.net/package/disk/misc/fixhddsizeRegarding SFS, you can install it from everywhere. I am sure there is documentation somewhere.
Short version: run HDToolbox, select drive, goto partition drive, add/update, add new, select smartfilesystem file, overtype identifier by SFS dostype (0x53465300 IIRC), click ok, select a partition, click on change and change file system to SFS. Repeat the latter for each partition.