Baby slept through feeding

My baby is a month old and usually eats every 3 hours or so. He is formula fed for now, (still working on building supply but that's another topic) The last two nights he has eaten extra, 5-6 ounces instead of 4 and he falls asleep. Then he slept through his next feeding. I love that he is letting me get some sleep but is it ok to let him sleep that long in between feeds? His next doctor appt isn't for another month, so do I track his weight myself to make sure he's gaining? Maybe I'm just lucky he's able to go longer periods at night now:)