>The PIC is operating in HS mode with a 4MHz oscillator.

Are you sure you should be using HS mode? The data sheet lists XT mode as
being correct for 4MHz. This may not be what is stopping your application,
but you could be damaging your crystal by overdriving it.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.