Doesn't sound like any problem here to me. Could baby be teething that's why is is not interested or cannot sustain long interest at the milk bottle. Some babies teeth at 4 months and signs can appear up to a month before that. My boy also mix feeding that time and sometimes I need to make him drink otherwise he won't drink - if he doesn't see the milk, he won't cry for it; if he sees it or drink a bit, he will wana continue drinking lots
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