Aww first time mom worries talaga, nakakabilib! Black or dark poop at 4 months can be normal depending sa diet niya (like kung may iron-fortified formula or kung nag-start na kayo ng solids), pero better check with your pedia just to be sure kasi mas peace of mind mo yan, sometimes dark poop is nothing, but your doctor can give you proper assessment in person dahil nakikita nila talaga ang baby mo.