Marian: Wow, sounds like you have an angel baby... and one night waking at that age is perfectly fine. If you feel your lo is getting enough and doesn't require it, that's your choice, but I think most los still get at least one, if not two night feedings at that age. That's waht I've noticed on this forum anyway.
Around 4 months, my lo did adjust to about a 4 hour EASY, but it's different for everyone.
If you feel your lo is ready to start stretching her times, go for it. But I would advise as the last poster, follow her lead. I found my lo did the same as yours, started out in the morning doing 4 hours and by night it was back down to 3. Eventually it all sorted out. But I never stressed 4 hours each time. I just went with waht my lo needed.
Your supply will not decrease at all. No need to pump or anything. Your body will just adjust to the new demands. Boy, our boobies are amazing things! lol
HTH