My baby is 4 months old & coos & laughs. My friends baby is 3 months and says Dada and mama. Is my baby not hitting his milestones?

You baby is fine. Your baby is just fine. Your baby is doing exactly what is expected at that age. All babies develop at different rates, so I wouldn't worry at all. Most babies will say mama and Dada between 4-9 months.
Baby milestones. From what you describe your baby is very normal. Google baby milestones to educate yourself so you can stop worrying to much.
Milestones OK. Your baby at four months is hitting his milestones, cooing and laughing is just right for a 4 month old baby! He soon may start to babble and imitate sounds. In my 19 years of seeing pediatric patients I have never seen a 3 month old that said "mama/dada", and it would certainly not be expected! It's so difficult not to compare your baby to your friend's! Try this site to track milestones! http://www.healthychildren.org/English/ages-stages/baby/Pages/Developmental-Milestones-3-Months.aspx.