Ay, that's a good question, normal breathing for a 4-month-old is usually around 25-40 breaths per minute when they're at rest, but honestly, it can vary a lot depending kung matulog sila or aktibo lang sila. I'd really suggest checking with your pedia though para ma-check properly, especially kung concerned ka kasi they can listen and give you peace of mind, one of my mum friends was worried din before at ang doctor assured her na normal lang ang baby niya, so it's worth the visit!