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:

fsck.ext2: Invalid argument while trying to open /dev/md0

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 <device>

or

e2fsck -b 32768 <device>

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

If I run cat /proc/mdstat I get:

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]

md0 : inactive sdb[1](S) sdc[2](S) sdd[3](S) sda[0](S)

1172054204Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]

md0 : inactive sdb[1](S) sdc[2](S) sdd[3](S) sda[0](S)

11720542048 blocks super 1.2

unused devices: <none>

8 blocks super 1.2

unused devices: <none>

Lastly running file -s /dev/md0

I get:

/dev/md0: empty

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?