First thing to do is to figure out where the alternate superblocks are stored on the slice. This can be done using for example command: newfs -N /dev/ad0s4a. The -N options says that the newfs doesn’t really do anything just shows what would be done. The output will look like this:
/dev/ad0s4a: 1024.0MB (2097152 sectors) block size 16384, fragment size 2048
using 6 cylinder groups of 183.77MB, 11761 blks, 23552 inodes.
super-block backups (for fsck -b #) at:
160, 376512, 752864, 1129216, 1505568, 1881920
After you have the super-block backups run the fsck manually fsck_ufs -b 160 /dev/ad0s4a. You can try all the backups by giving each after the -b option one at a time.