mdadm: add new device failed for /dev/hdc1 as 4: Invalid argument??

[Dr.Diesel]

Attempting to add a disk, then grow the array! Any ideas?

/dev/md0:
        Version : 00.90.03
  Creation Time : Mon Mar 13 16:46:02 2006
     Raid Level : raid0
     Array Size : 673990528 (642.77 GiB 690.17 GB)
   Raid Devices : 4
  Total Devices : 4
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Mon Mar 13 16:46:02 2006
          State : clean
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0

     Chunk Size : 128K

           UUID : ce7be438:db29be97:47f7ba78:1cf3c50d
         Events : 0.1

    Number   Major   Minor   RaidDevice State
       0      33        1        0      active sync   /dev/hde1
       1      33       65        1      active sync   /dev/hdf1
       2      34        1        2      active sync   /dev/hdg1
       3      34       65        3      active sync   /dev/hdh1

Adding code used, tried a couple different methods:

mdadm: add new device failed for /dev/hdc as 4: Invalid argument
[root@localhost garage]# mdadm --manage --add /dev/md0 /dev/hdc1
mdadm: add new device failed for /dev/hdc1 as 4: Invalid argument
[root@localhost garage]# mdadm /dev/md0 -a /dev/hdc1
mdadm: add new device failed for /dev/hdc1 as 4: Invalid argument

FC6 fairly fresh install.

Thanks,
Andy