For a Raid 10 con-fig, do the drives have to be the same model or can they be just the same manufact. and size?

I would like to have a Raid 10 array. I understand that you have to have at least 4 of the same drives. I currently have two 500GB Western Digital drives. Can I just buy any two WD 500GB drives or do they have to be the same model number as the two I already have?