ok, first, it's ok for breastmilk to pool in a baby's mouth during the night, it actually kills the bacteria that cause cavities. It's formula and cow's milk that are bad for teeth at night.
It's time to nightwean, and shouldn't be too hard. Basically, Dad takes over at night until he can fall asleep without it. Stand firm in denying it to him, even if you have to lie about it, "mommy's milk is all gone after 9pm," for instance. This is a good time to start setting boundaries about where and when you two will nurse. I stopped nursing in public after about 18mo and went with what *I* was comfortable with after that (but not to their detriment, of course).
I didn't have to wean to get pregnant, and my period returned after about 6 months, and I nursed exclusively, not even introducing solid food until they could chew it, around 7-9mo, depending on the kid. If nightweaning doesn't bring back your period, and you CAN still be fertile without a regular period, go see your OB.
And hurray for four! The fun of a big family, and everyone has someone to play with :)