How much should a 4 month old weigh?

My baby is 4 months old (turned 4mth on 18 june). She is now 5.5 as of today. It's always been my biggest concern. Even this app says, 6.6kg to 7kg. Is she underweight? She always only breastfeed for 5 mins on only 1 side for each feeding. I try my best to make her get a good latch. Pls help mommies!