There are multichannel buck PWMs (some w/ integrated
power switches) from LT/ADI and TI/National. But I am
not going to recommend a particular one since you say
not one thing about attributes beyond "4", and I have no
direct experience with any of them anyhow. Seems to me
that you would be best served by a distributor's component
selection utilities, on their web sites (what you can get,
trumps what you might like).