Some say to start introducing food to my baby at 4 months and some say start at 6months. which is the best and what's the reason?

4-6 months is Recomm: Baby's do fine with nothing but breast feeding the 1st 6 months.I do start cereal at 4 months to teach the baby "how to use " the spoon, but nutritionally it is not necessary and may limit or interfere with breast feeding.

No hurry: The main goal of introducing solids today is to help baby learn a new feeding method & gradually get them to feed from the table. By 6m most have lost their tongue thrust & can sit & learn.A simple rice cereal made up as a paste is a good starter food & should be offered from a spoon, not in a bottle. Gradually introducing one new food every few days to a week, allows you to monitor.