How much should i be feeding my baby?!

My baby’s 4months & I’m gonna start her on baby cereal the rice & oatmeal one. She drinks 4oz how much do i put in 4oz? Also.. she eats 4oz every few hours maybe 4hrs or so.. how can i get her to eat more for her age she should be eating 6oz every 3 hours or soooo. She weighs 12lbs yesterday when we weighed her and i know she should be 14lb for her age group please help 😬