Similar to free inodes, you should also check for maximum number of directories. dir_index option helps, but I've seen it become a problem.
Where I saw it crop up was 32K folders under /tmp on a cluster system. So no it's not a limit on number of directories entirely (that's inodes), but rather how many subdirectories you can have.
http://en.wikipedia.org/wiki/Ext4#Features <-- Fixes 32K limit