Note that FreeBSD 8.4 is from the older stable release branch, and 9.2 will release from the more recent stable branch with a lot of additional functionality and enhancements.

The topic of the zpool version for 8.4 is covered directly in the release notes:

FreeBSD ZFS subsystem has been updated to support feature flags for ZFS pools (the SPA version is 5000). Asynchronous destroy of ZFS dataset, LZ4 compression, ZIO NOP-write optimization have been implemented as new features. loader(8) tunables vfs.zfs.sync_pass_deferred_free, vfs.zfs.sync_pass_dont_compress, vfs.zfs.sync_pass_rewrite, and vfs.zfs.nopwrite_enabled have been added.[r243717, r244088, r247310]

Note that this upgrade can cause interoperability issues when upgrading a FreeBSD 8.4 system to 9.0 or 9.1. This is because FreeBSD 9.0 and 9.1 support SPA version 28 and do not recognize version 5000. To mitigate this issue, the default SPA version for a newly created ZFS pool on FreeBSD 8.4 is set to version 28. To create a ZFS pool with version 5000, use zpool(8) upgrade command after the creation.

If it has LZ4, I don't think it has a zpool version. Versions were deprecated, and now ZFS uses feature flags instead.