Mama, I know how stressful this is, especially as a first-time parent, but what you're describing actually sounds quite familiar to a lot of us who have been through the 4-month stage. Around this age, babies go through a developmental leap that can really throw off feeding, and some also start getting distracted or developing a stronger preference, so the low intake during the day with one better night feed is a pattern a few mums I know have seen too. It is worth checking whether your bottle nipple flow size needs to go up, because if the flow is too slow it can frustrate them and cause them to pull off and cry mid-feed, sizing up made a noticeable difference for some, though a small number found it caused more gulping and wind, so it is a bit of trial and error. My friend actually brought her baby to a lactation consultant when this happened, and she said the consultant helped rule out things like tongue tie and also guided her on paced bottle feeding, which calmed the whole feeding experience down quite a bit. That said, 400ml at 6.8kg is on the lower side and the struggle and crying during feeds is worth taking seriously, so I would still bring it up with your PD sooner rather than later to make sure there is nothing else going on.