An existing mdadm RAID5 is not mounting, Either a problem drive or Superblock

I have 4 x 3TB NAS setup as RAID5 which has been working great for almost a year.

After a recent abrupt shutdown (had to hit the power button) the RAID will no longer mount on boot up.

I've run:

mdadm --examine /dev/sd[bcdefghijklmn]1 >> raid.status

The output is below:

/dev/sda: Magic : a92b4efc Version : 1.2 Feature Map : 0x0 Array UUID : 7d2a94ca:d9a42ca9:a4e6f976:8b5ca26b Name : BruceLee:0 (local to host BruceLee) Creation Time : Mon Feb 4 23:07:01 2013 Raid Level : raid5 Raid Devices : 4

Avail Dev Size : 5860271024 (2794.40 GiB 3000.46 GB) Array Size : 8790405888 (8383.18 GiB 9001.38 GB) Used Dev Size : 5860270592 (2794.39 GiB 3000.46 GB) Data Offset : 262144 sectors Super Offset : 8 sectors State : active Device UUID : 2c1e0041:21d926d6:1c69aa87:f1340a12

Update Time : Sat Dec 27 20:54:55 2014
   Checksum : d94ccaf5 - correct
     Events : 17012

     Layout : left-symmetric
 Chunk Size : 128K

Device Role : Active device 0 Array State : AAA. ('A' == active, '.' == missing) /dev/sdb: Magic : a92b4efc Version : 1.2 Feature Map : 0x0 Array UUID : 7d2a94ca:d9a42ca9:a4e6f976:8b5ca26b Name : BruceLee:0 (local to host BruceLee) Creation Time : Mon Feb 4 23:07:01 2013 Raid Level : raid5 Raid Devices : 4

Avail Dev Size : 5860271024 (2794.40 GiB 3000.46 GB) Array Size : 8790405888 (8383.18 GiB 9001.38 GB) Used Dev Size : 5860270592 (2794.39 GiB 3000.46 GB) Data Offset : 262144 sectors Super Offset : 8 sectors State : active Device UUID : a0261c8f:8a2fbb93:4093753a:74e7c5f5

Update Time : Sat Dec 27 20:54:55 2014
   Checksum : 7b84067b - correct
     Events : 17012

     Layout : left-symmetric
 Chunk Size : 128K

Device Role : Active device 1 Array State : AAA. ('A' == active, '.' == missing) /dev/sdc: Magic : a92b4efc Version : 1.2 Feature Map : 0x0 Array UUID : 7d2a94ca:d9a42ca9:a4e6f976:8b5ca26b Name : BruceLee:0 (local to host BruceLee) Creation Time : Mon Feb 4 23:07:01 2013 Raid Level : raid5 Raid Devices : 4

Avail Dev Size : 5860271024 (2794.40 GiB 3000.46 GB) Array Size : 8790405888 (8383.18 GiB 9001.38 GB) Used Dev Size : 5860270592 (2794.39 GiB 3000.46 GB) Data Offset : 262144 sectors Super Offset : 8 sectors State : active Device UUID : 9dc56e9e:d6b00f7a:71da67c7:38b7436c

Update Time : Sat Dec 27 20:54:55 2014
   Checksum : 749b3dba - correct
     Events : 17012

     Layout : left-symmetric
 Chunk Size : 128K

Device Role : Active device 2 Array State : AAA. ('A' == active, '.' == missing) /dev/sdd: Magic : a92b4efc Version : 1.2 Feature Map : 0x0 Array UUID : 7d2a94ca:d9a42ca9:a4e6f976:8b5ca26b Name : BruceLee:0 (local to host BruceLee) Creation Time : Mon Feb 4 23:07:01 2013 Raid Level : raid5 Raid Devices : 4

Avail Dev Size : 5860271024 (2794.40 GiB 3000.46 GB) Array Size : 8790405888 (8383.18 GiB 9001.38 GB) Used Dev Size : 5860270592 (2794.39 GiB 3000.46 GB) Data Offset : 262144 sectors Super Offset : 8 sectors State : clean Device UUID : 81e5776f:2a466bee:399251a0:ab60e9a4

Update Time : Sun Nov  2 09:07:02 2014
   Checksum : cb4aebaf - correct
     Events : 159

     Layout : left-symmetric
 Chunk Size : 128K

Device Role : Active device 3 Array State : AAAA ('A' == active, '.' == missing)

When checking the Disks in Ubuntu Disk Manager sda/b/c are are showing as OK and sdd is showing as OK with 64 bad sectors

If I run fsck /dev/md0

It reads:

The superblock could not be read or does not describe a valid ext2/ext3/ext4 filesystem. If the device is valid and it really contains an ext2/ext3/ext4 filesystem (and not swap or ufs or something else), then the superblock is corrupt, and you might try running e2fsck with an alternate superblock: e2fsck -b 8193 or e2fsck -b 32768

Lastly if I run

mdadm --examine /dev/sd[a-d] | egrep 'Event|/dev/sd'

I get:

/dev/sda: Events : 17012 /dev/sdb: Events : 17012 /dev/sdc: Events : 17012 /dev/sdd: Events : 159

Basically I think I need to run --assemble on the RAID but I'm afraid of losing my data but also that 4th drive concerns me a little.

Could someone advise of the best next logical steps to get this up and running again?