Unless the rate at which I accumulate data increases significantly, 8tb should last me at least two years. By that time, either the cost of adding another 2 4tb drives would have dropped, or larger drives will be available for less. Adding drives would likely be cheaper (and provide larger space gains) than replacing drives by that point, though a little inconvenient as I'd have to offload the data before the upgrade.
The other thing that occurs to me is that I already have two 3tb drives. Buying another 4 would be cheaper than 4x4tb, and a 6x3 would give me more space (as Erinloewe has pointed out). It seems like this may be the best option?