I think AmigaOS 3.9 has drivers to support drives bigger than 4 Gig but I think if you store more than 4 Gigs on the partition it will overwrite the front of your partition when you get to the back of the first 4 Gigs.
No, it doesn't, at least if you have full OS properly installed (incl. ROMUpdate scsi.device).
The boot partition needs to be < 4Gig *area* of the disk since the system will initially start using ROM scsi.device which has no idea how to access >= 4Gig.
Once the setpatch has loaded the ROMUpdate (new scsi.device and fastfilesystem etc), the system is able to access arbitraty disk size (at least upto 127 gigs), and partitions can be larger than 4gig. Note that accessing these >= 4Gig area partitions is only safe when ROMUpdate is loaded properly.
The "wrapping" you talk about doesn't happen inside the partition. In case you use old scsi.device and try to write past 4gig border, it will not understand the 64bit offset and will wrap to beginning of disk, thus overwriting the RDB and first partition(s). So it is not triggered by partition size, but absolute write offset from the start of the disk. It is well possible to trigger the problem with 5 MB partition placed so that it overlaps the 4Gig border.
There are also application level incompatibilities. For example old disk repair/recovery tools won't understand partitions >= 4Gig border. They might also fail to handle >= 2Gig partitions. Also some old programs have problems with very large partitions, for example in "disk free storage" check (they might warn about lack of disk storage when the disk free storage is close to 2GB or 4GB multiples).
So to my knowlege OS 3.9 should work with >= 4Gig ffs partitions.
