My Baby Is 6 Weeks Old. He Has Not Pooped

My baby is 6 weeks old. He has not pooped for the last 4 days. Earlier he used to poop twice daily but suddenly this happened. I tried giving him juice of dried black seeded grapes but it dint work. He is playfull contently drinks breastmilk. What should i do. Is it a matter of concern.plz help