Alternating breasts/ sleeping thru the night

At my baby’s four week appt Today she weighed 13 lb 12 oz!!! So my dr told me to let her sleep as long as she will overnight (up until now I’ve been waking her every three hours to feed). Im planning not to let her go more than six hours but I’m worried about engorgment or milk supply issues. So far I’ve had problems with an overactive letdown so I always nurse on one breast per feeding because she doesn’t nurse for long. Like tonight I fed her at 7 pm on my right, 10 pm on my left but then If she does go until 4 am (which sounds way too good to be true) and feed again on my right then my alternating breasts may not get drained for 8 or 9 hours. I haven’t pumped yet as I’m worried about over supply but should I be in this situation? Or just express some milk as needed? Any of you have advice?