Strange feedings

Nn

Im a fist time mom and My baby boy is 4 weeks today and since yesterday during one of his feeding he only took 2 oz all other feedings he usually drinks 3 or 3.5 oz and just now during his feeding again only 2 oz ....is this ok ? he feeds both breast milk and formula he wakes up on his own every 3 hours to feed and sometimes cluster feeds so I have never had to wake him to feed