The maximum MCLK frequency for the AD7705 is 2.5MHz. It will operate with a
'slightly' higher clock frequency and you can expect an improvement in noise
performance (not guaranteed though), but up at 4.9MHz, you are exceeding the
limits of the part, it may be functional but internal timing, and performance
is not guaranteed and you might actually see an increase in noise due to clock
feed-through and/or lack of settling time.