theta0 is not a keyword in the function:
ChisqTest([25, 25, 25, 25], [0.25, 0.25, 0.25, 0.25])
works.
Note that the standard function used for calculating theta0 in the function is ones(length(x))/length(x) which would be [0.25, 0.25, 0.25, 0.25] in this case anyway, so you can do
ChisqTest([25, 25, 25, 25])
For the same result.