There's nothing built into PPT to help with this other than manual tedium.
Doing it via VBA is a possibility. The code would need to know where the boundaries of the current "slides" are within each slide, then look at each shape on a slide, work out which of the four "subslides" it belongs to, and copy the correct bits to four
new slides.
You'd want to make the new slides in a separate presentation that's sized to match the subslides on the four-up pages rather than any of the default sizes. Otherwise, the code would get bogged down in a scaling mudpit, since scaling the shapes wouldn't
scale the text.
If you want to tackle this yourself, we can help. If you want to hire it out, a couple of us do that as well.
If for some reason you HAVE to have the final slides at a particular size, I've got an inexpensive add-in that'll automate it (Windows only, though). http://www.pptools.com/resize/