Your baby is actually right in the average weight range for 4 months old. A baby boy of his age is on average 15 pounds. That means that while 50% of baby boys in the U.S. may be 15 lbs, there will be 25% that weigh more and 25% that weigh less. Your baby is ONLY 1 lb less and if he's in the 25%-tile he's still in the low normal average.
You have to take into consideration that he probably needs some time to adjust to the daycare. You also should take into consideration family genetics. What you really need to look at is if he's happy and healthy and thriving, not his weight. Is his color good? Is he developing typically? If you can answer all of those with "yes" then he's perfect the way he is.
What does your practitioner say? Are you getting pressure from someone who thinks that "all babies must be chubby" and if they're not in the upper percentiles then they must not be thriving? Are you comparing him to other babies in your group of friends or the daycare? Trust that he's perfect. Don't let friends and family criticize because they think he should be fatter because he's a boy or whatever the reason is.
I used to get flack from all sides with my girls. They weren't chubby enough to some people and were too fat to other people. Girls shouldn't be so tall, said some, yet to others my girls are all so dainty and short. I stopped letting what others said matter. Let them worry about their own kids. My kids are perfect little snowflakes. :-)