I skipped cereal too and started my LO on sweet potatoes- thanks to advice from an osteopath who is a father to 3. I just used timing to determine when my LO was ready. (Some books say to start just before 6 mos to help LO learn how to handle foods other than BM.) He was always interested in anything we put into our mouths and has shown a great interest in food. I kept him on BM until 5 1/2 mos then started solids- and it has been a journey.
4 months may be a little young, so nuse your nursling until you really think he is ready!
Chewing the hands is probably just teething!!! My 8 1/2 mo old favorite teethers have always been his hands! Despite urging him to use other objects/toys/teethers!