Hi Owen,
The sampling rate is limited by the processor bandwidth, so for both devices (4ch and 8ch) the maximums are:
#Channels...Max Sampling Rate
1-2ch......... 100kHz
3-4ch......... 50kHz
5-8ch......... 20kHz
It doesn't matter which channels you use, so long as you only have 1 or 2 in active playback at the same time you can play at 100kHz.
-Josh