Well it really depends on how much smoke you like. If you've cooked before and you have a sense of how much wood/smoke you like, then I would say stick with that amount. The fact that you're doing 4 butts doesn't increase the amount of wood needed.
If, as you indicate, you only have hickory, then it depends on what you're use to mixing it with. If you typically mix a hickory with a light wood like apple, then I would substitute hickory to apple by using a little less hickory than apple. If you typically mix hickory with a stronger wood like oak, then I would substitute hickory to oak by using a little more hickory than oak.
Personally a couple fist size chunks is good for me, but I'm a light weight. Remember - going light on smoke might cause a little disappointment in the finished product, but going too heavy can down right ruin a piece of meat (or four pieces in your case).