Hi John,
Encoder signals do not have any 4us requirement. They are basically A/B square waves not pulses. The max count rate for the encoder inputs are specified at 1 million quadrature transitions/second (250KHz cycles for both A and B). The encoder signals are sampled at 16.67MHz and pass through a programmable digital filter to eliminate noise glitches. The default level of filtering is 7 which means the A/B levels must be continuously stable (high or low) for 7/16.67 = 0.42us to be considered a valid transition. The range for the digital filtering is from 1 - 255.
HTH
Regards