Is it normal that my baby only eats 3-4 oz still?

She's 4 months next week and still only has 3-4 oz at a time, every 2 to 3 hours. SOMETIMES more. Exclusively formula fed  
she's been drinking 4 oz for three months now... I thought she would've started drinking more by now?? Is this normal?? I don't want to underfeed her. My friends baby the same age drinks 8 oz at a time 😳😳