no BM since Sunday?

Courtney • Gave birth to the love of my life on 9/21/2016 😍😍. I’m due with baby #2 9/17/18!! I can`t love anything or anyone more than I do my babies 💙💙💙
So my almost 4-month old son hasn't pooped since Sunday, New Years Day.  He has been exclusively breastfed since birth. He has had his fair share of wet diapers, isn't fussy or crying from discomfort, his belly is soft, sleeps through the night and he is still eating every 3-4 hours.  When should I be worried?