Hi pn2543-
The 4-bit resolution specification you mention refers to a 4-bit frequency divider onboard the device that can be used for pulse-train generation, using the 10MHz or 100kHz timebases on the board. In NI-DAQmx, this output is referred to as "freqOut".
The analog input sampling rate is actually determined as 24-bit divisions of the 20MHz (or 100kHz timebase, if needed) on the device. The maximum sample rate is 500kS/sec for single channel according to the specs, and the minimum would thus be 100k / (2^24-1) = 0.00596 S/sec.
Hopefully this helps-