Seems like the copy of Hydra in the BT4 pre-final is still lacking SSH2 support.
# hydra -s 22 test.com ssh2
Error: Compiled without LIBSSH support, module not available!
A fix for this was posted in the BT4 beta bugs & fixes forum (can't post links yet), but it would be nice to have native SSH2 support in the final release.
[Update] This fix did not seem to work for me... see below.