I just created a 4 drive mdadm --level=raid10 on a centos 4.8-ish system here, and shortly thereafter remembreed I hadn't updated it in a while, so i ran yum update...
while installing/updating stuff, got these errors:
Installing: kernel ####################### [14/69] raid level raid10 (in /proc/mdstat) not recognized ... Installing: kernel-smp ####################### [19/69] raid level raid10 (in /proc/mdstat) not recognized
is this a problem? the raid10 'seems' to work....