4 serial inputs 1 chip

[dhouston]

Thanks Dave but the design must be implemented with PIC. But I will check on your chip recommendation for future designs

Then you'll need to look at Darrel's Instant Interrupts.

Or, if you don't have to worry about simultaneous inputs, look at a DG409 and http://www.maxim-ic.com/appnotes.cfm/an_pk/588.