The recommendation is not to begin weaning until 6 months old, babies can easily survive on milk alone until then. Traditional signs that a baby is ready to start weaning (stopping sleeping through the night, putting things into their mouths, showing interest in food etc) do not necessarily mean that a baby is ready for solids. There is a big growth spurt at 4 months which generally just lasts a week or so but can be mistaken as a baby needing solids. Babies will also show interest in anything you do, including eating. Chewing on hands etc is a developmental phase.
Personally I wouldn't wean before 6 months unless it is on the advice of a doctor. I weaned at 5 1/2 months on my doctor's advice and am still not convinced she needed to be weaned at that stage!
