matlab. vectorization within if/else if/else statements

matlab. vectorization within if/else if/else statements

I need some help with the following code:
if x(:,3)>x(:,4)
output=[x(:,1)-x(:,2)];
elseif x(:,3)<x(:,4)
output=[x(:,2)-x(:,1)];
else
output=NaN
end
Here is a sample data:
matrix x output
10 5 1 2 -5
10 5 2 1 5
NaN 1 1 3 NaN
I'm not sure how to make the code work. It just takes the first argument
and ignores the else if and else arguments. Please help. Thank you.