My 4 month old seems like he has difficulty breathing, don't think he uses his nose, he gasp for air when he is feeding, could there be a constriction?

Bugger blockage: Infants at that age are dependent on clear nasal passages to channel air to their lungs. They need another month or 3 to grow a neck long enough to comfortably drop their jaw & mouth breathe. During early infancy, dust, perfumes, odors all make babies nasal passages swell & their secretions thicken. Copious saline nasal wash is often necessary to keep their nose clear & parents seldom do it well.

Maybe: Has his breathing pattern always been like this, or is it a recent change? If recent, does he have a cold? Try putting a few drops of saline in his nose, and suction with a bulb syringe, especially before feeding or when he seems congested. If he has always breathed like this, have your pediatrician examine at rest and feeding. Choanal atresia is congenital stricture of nose, needs ent.