This is a very rudimentary command-line tool that can read ext4 file systems. You can either point it to a /dev/sdXX file, or a file you created using the mke2fs command.
It does support flexible block groups.
Note: I am working on this tool primarily to recover data from a filesystem that was spoiled by a failed GParted partition-resizing operation.
This tool expects the path of the file system, along with its size (in bytes).
./reader.py -f <filesystem> -s <size> -r <yes|no>
Beware that by using the recovery option, you get a dump of the contents of all the files on the file system.
I've tested it only on Ubuntu 14.04.