Back to the OP and the baby not crying to be fed. Sometimes you'll have babies that don't cry for food.Especially babies separated from their siblings. usually it because they never completely void their crop of food. Competition between babies can be a good thing. You should be trying to stimulate them into begging before feeding them. During the night their crop should completely empty. It could be yeast infection that builds up. It could be crop impaction from undigested food. And it could be from feeding too thick of formula,too cool, too often. The begging,crying is a normal response and i suspect there's sometime wrong. Your 4wk old zon should be eating more than 9 mils @3 times per day. What weight data do you have? Is there a trend?