I only BF'ed my 3 DCs for a few weeks/months each so I have no idea what happens in this situation. If you are still feeding a baby or a toddler and you get pregnant what do you do? Are you advised to stop, or does it make no difference to the unborn baby's health or yours? And if you do continue, what happens when you have the baby? Do you manage to feed both? How do you know the new baby is getting all the colostrum? 
And if you make the older child stop feeding in order to exclusively feed the new baby does it cause jealousy/anxiety for the toddler?
Sorry for such a dumb random question, but it just popped into my mind and I have to know the answers!
I realise BFing is supposed to act as a natural contraception, but it is not especially reliable, is it?