Ext4 superblock corrupted after successful(?) mdadm grow and resize2fs

You might want to take a look at ddrescue now to see if anything at all can be recovered...

http://www.gnu.org/software/ddrescue/ddrescue.html

I'm thinking you jumped the gun and resized the fs before it was finished. From the dmesg output you can see that it had to finish the reshape after you rebooted:

[    9.542918] md/raid:md0: raid level 5 active with 4 out of 5 devices, algorithm 2 
[    9.543005] md: recovery of RAID array md0
[    9.543007] md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
[    9.543008] md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for recovery.
[    9.543013] md: using 128k window, over a total of 488382784 blocks.
[    9.543014] md: resuming recovery of md0 from checkpoint.
[ 4208.380943] md: md0: recovery done.

Edit: the data is most likely gone.