A comment helped me to fix this, but I want to write up an answer in case anybody else runs into this issue.

An ode function should take only two parameters: time and a vector with any other variables. The following modification fixed my issue:

function [dndt,dmdt,dhdt,dVdt]=hh(t,x)
    n=x(1); m=x(2); h=x(3); V=x(4);