No, I don't know how much tincture you'd get with the 4 or 8oz. but I would at least order the 8oz. just to be safe. I have no idea what size jar you're going to be using to tincture. I use only 1/2 gallon jars and with that size I use about a cup of each of the Echinacea's but I've never weighed those cups to see how much they weigh, might be more than 8oz, just don't know.
Yes, you want both the Angustifolia and the Purpurea root for the tincture.