My 4 year old is not on growth chart he was born 6 lb but 2 week later drop below 5 percentile and never recover. Should I be concern?

Growth chart is it! To many factors to consider, but growth potential (parental height), nutrition and underlying illnesses (ie. Heart, metabolic, gastrointestinal, genetic, etc) may play a role. Talk with your doctor and ask to see the growth chart. In it is the key to being reassured or finding reason for further work up.