>>What cabinet volume would be appropriate, or how can I calculate it?

Follow the link and put in the parameters of the driver in the database maintenance and run the program.

>>I am looking at 3/3/6/8 arrangement for the 4" and 5/5/6/6/8 for the 3/8". Is this proper tapering?

Yes and you will need to do it, I have to do this on my tweeter array for sure, I discovered that when I was testing the array.

>>Would a ported cabinet work better?
Somewhat depends on where your x-over point to the sub is, but your still not going to get much below 100Hz. And with the somewhat hi Qts of the drivers it'll require a larger cabinet.

HTH
Bill W.