Formatting was much faster (less than a minute) for the ffs compared with fat32 (more than an hour). This is surprising because you mentioned that every filesystem needs this extra stuff like lists of free space which would take time to put on the disk.
The result
Code:
# disklabel sd0
# /dev/rsd0c:
...
16 partitions:
# size offset fstype [fsize bsize cpg]
c: 7814037167 0 unused
d: 4668308928 3145728064 4.2BSD 8192 65536 1
i: 3145728000 64 MSDOS
# mount
...
/dev/sd0d on /mnt/sd0d type ffs (local)
/dev/sd0i on /mnt/sd0i type msdos (local)
# df -h /mnt/*
Filesystem Size Used Avail Capacity Mounted on
/dev/sd0d 2.2T 8.0K 2.0T 0% /mnt/sd0d
/dev/sd0i 1.5T 8.0K 1.5T 0% /mnt/sd0i
Quite some overhead (0.2 TiB) for the FFS.
Of course the total does not add to 4 TiB but rather something like 4 TB, because hard disk vendors use base 10 (or, alternatively, they are bastards).
As mount(8) does not show it is FFS2 and the next thing I saw was that I have "2.0T" available on the FFS filesystem, I thought it was FFS, but I guess it is FFS2.