Quad voltage controlled resistance
First, the \$R_{DS(ON)}\$ vs. \$V_{GS}\$ of a FET depends on a number of quantities which are not normally defined in the datasheet, and which vary from part to part and over temperature.  There is not a nice linear, unchanging relationship between \$V_{GS}\$ and \$R_{DS(ON)}\$.

Second, your FETs will never be matched close enough, probably not even if they all came on the same die.

Third, you're not really implementing an audio pot, because the resistance across the assembly won't be constant.