Greg,
as Steve has already mentioned: Opto-Isolators are perfect.
the internal PullUp is too weak for this.
You should definitely use ane external one and this should be less than 10k, say 4k7 or 2k2 or even lower..
You could use one resistor at ech end of the line.
bear in mind that with resistors at both ends of the line
you would have 8 resistors in parrallel between VSS and VDD, when all 4 switches are closed.
For 2k2 Resistors this would mean 18mA. This could be an issue for a battery powered application.
For a simple filter a 100nF Cap and a 10k Resistor (close to the PIC) would do.
regards
Ralph
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