My son is four years older than my daughter and I breastfeed them both as well. He was so curious about everything to do with the new baby, just like your son.
When he asked this question, I explained to him that God has provided the perfect food for each of His creation. And that just like puppies and kittens (that he had seen nurse) that God had provided for babies as well.
I explained that breastmilk was perfect for their little belly and that it had lots of good things in it that helped babies grow big and strong, just like he had. :-)
But it was only for babies and that God prepared babies for big boy food when babies start to get teeth. Through our discussion, my son realized that I was going to be doing for his new sister just what I had done for him.
As curious as my son was about the new baby, he also needed a lot of assurance that his place was secure in our hearts.
We tried to include him as much as possible in the daily routine of taking care of his new baby sister. We also made sure that we set aside special time just the two of us, so that it wasn't such a hard adjustment for the little guy, who until his sister showed up - had our full undivided attention.
During this time, one of my dear friends gave us a baby shower and along with the new gifts for the baby, several of the ladies provided small gifts for my son. I had never heard of this before, but since then have included a small separate gift for new siblings when I've given a baby gift.
The new brother or sister think this is so cool! Everyone is so excited about the new baby that new brothers and sisters sometimes get shoved into the shadows.
God bless you and your family. Twins and a four year old. Wow, that's a blessing in itself.
Hope that helped.