There's really no point. At 4 months, just up the amount of formula or breastmilk he is getting. The size of your son really doesn't make a difference at this age...he's getting everything he needs from breast milk/formula.
I started solids at 4 1/2 months due to reflux, but if it hadn't been for that I wanted to wait to 6 months. It can cause food allergies to introduce solids too early. If you choose to do so, realize that you do not have to start with rice cereal. I started with sweet potatoes, then did green beans, then started fruits. you can start straight away with purees...rice cereal may be linked to eczema, and really doesn't have much nutritional value.