I'm feeding my baby 4 oz. of formula every 3 hours. if she doesn't wake up before then during the day, do i need to wake her up? she sleeps after.

Depends on her age: If your baby is less than a month old, I would recommend trying to feed her on a semi-regular basis every 3-4 hours. She may not realize that she's hungry, and may sleep so long that she is famished when she wakes. That may lead her to try to overeat. After the first month, I usually let the baby set the schedule, sleep longer through the night, and make up for it during the day.