Eh don't worry lah, this is actually quite normal for breastfed babies especially at that age, some of my mum friends had babies who only went once a week and paediatrician said it's fine as long as baby is acting normal and perut tak keras! Since your baby is happy, kentut, and perut lembut, sounds like everything is moving through okay, but 4 days is getting a bit long so I'd suggest checking with your doctor just to be sure, better safe than sorry kan, and they can confirm if it's just the mix of BF + formula or if kena adjust something.