The error when using interpn is due to the data not being present in Grid format, meaning for every triplet of (x,y,z) in some range of x,y,z should have a value v also present. You can create the Grid format data for x,y,z using
[X1, X2, X3] = ndgrid(x,y,z);
But for the same you need to reshape the 4th column such that it represents corresponding values in X1, X2, X3. For this the length of unique x,y,z is required which gives the indices where values for learning interpolation are present.
You can see here about how the input array should look like.