The other thing is RAID 4 really hammers its parity drive. If you had just a 3 disk RAID 4, the parity drive gets hammered at least twice as much as the data drives during writing (particularly important if you are recording lots of TV), for a 4 disk arry, at least 3 times as much... it almost becomes self defeating in terms of redundancy. RAID 5 aleviates this issue, but both 4 and 5 have horrible write performance, especially for small and random writes. Dunno about mdadm but certainly most hardware RAID solutions can expand arrays semi-online... meaning you don't have to copy the data off, but you can't have the drive mounted at the time, and some can do it truly online.
In terms of separating tasks/responsibilities, I would suggest getting a NAS with RAID functionality, and divorce your data completely from your core/MDs, another option is FreeNAS software - Norcotek have some nice looking 5 bay NAS's that are compatible with FreeNAS, or simply build yourself a small/cheap PC with lots of drive bays and SATA connectors, and install FreeNAS on that.. can be a lot cheaper!