Hmmm, actually, in PWM mode the CCP module is run from TMR2 which is concatenated with the two bits from the internal clock divider (which is where the "extra" bits of resolution comes from) so each "dutycycle bit" represents one TOsc - not four. From the 18F25K22 datasheet;
PulseWidth = CCPRxL : CCPxCON<5:4> * TOsc * TMRxPrescaler value
So with a 64MHz oscillator it should be possible to generate 15.625ns pulses using the CCP module.

/Henrik.