I have a big boy too. We are at 7mths and 23lbs - don't remember what we were at at 4mths though.
It sounds like your little one is thriving well on breast milk, so it certainly wouldn't hurt for you to wait if you think that it isn't agreeing well with him. We waited until 6mths per the advice of the American Pediatric Association. Remember that even though he is getting cereal, that 90%-95% of his caloric intake should be breastmilk or formula until he is around 10 mths - everything up until then is really just practice.
Also, my doctor advised against feeding cereal in a bottle. I know that many people do, but one of the main points of introducing the cereal is to get them use to eating from a spoon.
Our son definitely went through a BM transition when we started solids, but he did not get constipated so I am not sure if this is a sign of him not being ready or not. Hmmmm, we didn't do this until our son had been on cereal for a couple of weeks, but, to help constipation you can add a little bit of pear juice to the mix.
Hope that helps! Good luck!