Can I solder 8-channel AD7606 in 4-channel AD7606-4 footprint?
It is possible to damage the aditional channels by conecting to GND.
This is the only difference of these ADC's.
The AD7606/AD7606-6/AD7606-4 are 16-bit, simultaneous sampling, analog-to-digital data acquisition systems (DAS) with eight, six, and four channels, respectively...
AD7606 on Analog.com