Sounds like you have a big project on your hands. Hope I helped, but others have more experience than me.
I have never tried this myself, but the usual procedure is to use one of the pics as the master, like Papa said, and that pic can accept inputs from the other 4.
This is what I would try (may not work).
4 16f877's, each a slave
1 Master PIC (another 877 may be easiest)
each slave PIC PortC wired together in one bus.
Master would have interrupt request inputs from each 877.
Master tells which pic can send data (PortC becomes output), otherwise all 4 877 PortC's are tri-stated or inputs.
While very overkill, I would use 5 pics just for the experience it would give me. It would be a rewarding project when finished. But an analog multiplexer, as Papa mentioned, would be the easiest. You would also have fewer forehead marks in your walls, too.