GPS accuracy vs RNP requirement
You're right, kijangnim, I mixed up the definitions: RNAV (RNP 5) must meet a track keeping accuracy equal to or better than +/- 5 NM (PRNAV=1 NM) for 95 percent of the flight time. It has nothing to do with GPS accuracy. So I gather the 4/2/1/.03 values are the full-scale deviations on the cdi, which become more and more sensitive as the a/c enters the terminal area and then to the approach.
What is still confusing to me is that the enroute RNP requirement is stated once as 5 NM and then 2 NM (I didn't find contradicting values for the other phases of flight except the enroute segment).