the longer the bulb the more phsphor effiecient they are.
i'd go 2x4 1x6 1x6 2x4
that way you could run the 4 4fts off an IC 660 and the 2 6fts off another IC 660.
I have an extra one at home and I don't need it right now.
I paid $20 on it. Send me a PM if you are interested and I will send you my phone number and my address.