M.B.
S.,
Awesome job breastfeeding! So, so good for your son, and you too.
You can increase your milk with Fenugreek and Blessed Thistle combined (I buy them separate). Mother's milk tea will help also to increase your supply.
He is only 4 months, he's still enjoying the wonderful immunity benefits (he'll have fewer runny/stuffy noses thanks to you, as well as fewer other sicknesses). The more he suckles the more milk you will produce. The more you supplement, the less you will produce.
Good Luck, and congratulations!
M.
http://breastfeedingonline.com/newman.shtml