AD7190 noise performance calculation
Since all of the noise specifications are listed as typical, it is not that critical as to how you model your system. In any case, I doubt that using a 4 volt reference instead of a 5 volt reference would make any difference as far as input noise is concerned since the reference only affects the digital part of the circuit. Grounding one side of the input will probably reduce the effective noise by a factor of √2 which, again, is not that all significant.