Also is it possible the four year old is wakign your baby? Or four in the bed is just harming everyone's sleep? When we had two little ones at the same time, the older one (at that point 3.5 or so) kept waking his baby brother up. So we transitioned our older child into his own room for naps and nighttime (or at least the first half of the night) at that point. And he was still nursing and continued to nurse.
FWIW here is my favorite short article on toddler nursing http://www.breastfeedinginc.ca/conte...agename=doc-BT