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.
Yes it is possible to solder the 8 channel version in place of the AD7606-4 as these devices are pin for pin compatible. Make sure to connect any unused channels to AGND.