OK, first of all, you can know if you are giving your baby enough nourishment by the amount of wet and dirty diapers and you may not have to give him formula at all if you don't want to. But that's all up to you. A great place to get all that info is at the La Leche League web site www.llli.org or Dr. Sears web site. But if you do want to supplement with formula for any other reason, it's going to take more than 4 days to see him level out so to speak. You were breastfeeding and in doing that, you were giving him something that is tailor made for his body and now you've introduced something that is a little tougher to digest. It's not bad, it's just making his body work harder. The cow's milk protein or soy protein is a little harder to digest and the body sees it as foreign until it gets used to it.
The other thing is that with the body getting this "foreign matter" in it's system, the (sorry if it seems gross) rectal muscles sometimes forget how to work or don't know how to work well enough to push through the waste. You can ask your Ped about that. And sometimes you may have to stimulate the rectum in order to get him to have a BM.
My son had a similar thing at the same age. I breastfed, but had to be hospitalized for what I thought was an appendicitis attack that turned out to be diverticulitis. I couldn't breastfeed for a few days because of the medications they gave me and because of the barium they had me drink for the CT scan. So we introduced formula. It was horrible, we thought he was constipated, gave him a little sugar water (under the Ped's advisement) and it would get better for that BM but go back the next time. His rectal muscles were weak and he needed to be stimulated. (your PEd can tell you what or if you should be doing this) It wasn't until 2 weeks later that we saw an improvement. I was able to breastfeed again, and his system re adjusted. We've since learned that he has a slight cow's milk allergy and he's definitely lactose intolerant.
Also, just so you know, our Ped had told us that with any change, formula or otherwise, it takes at least 2 weeks for the intestines to re adjust. So hang in there, visit the la leche league web site, they have great breastfeeding help and tips regardless of weather or not you are supplementing with formula. It's not a sin. ha ha You're just doing what you think is best for your baby.
You guys hang in there. This will pass. You're such a wonderful mama to be looking for advice so that you can do the best for your little guy. He's very lucky indeed!
Hugs!
Kel