Recovering ext4 superblocks

Unfortunately, I was unable to recover the file system and had to resort to lower-level data recovery techniques (nicely summarised in Ubuntu's Data Recovery wiki entry), of which Sleuth Kit proved most useful.

Marking as answered for cleanliness' sake.

This may be outdated already, but a few suggestions:

If you are absolutely sure that the original blocksize is 4096, as claimed by testdisk, you can rewrite the superblocks on the disk using mke2fs -S. From man:

   -S    Write  superblock and group descriptors only.  This is useful if all
          of the superblock and backup superblocks are corrupted, and a  last-
          ditch  recovery method is desired.  It causes mke2fs to reinitialize
          the superblock and group descriptors, while not touching  the  inode
          table and the block and inode bitmaps.  The e2fsck program should be
          run immediately after this option is used, and there is no guarantee
          that  any  data  will be salvageable.  It is critical to specify the
          correct filesystem blocksize when using this option, or there is  no
          chance of recovery.

If you are unsure about the correct blocksize, use mke2fs -n -b 2048 /dev/sdb1 and try all the superblock backups this command gives, and after that the same but using the last blocksize 1024.