4D interpolation

Hi
I want to interpolate my values with the spline method. I did it with the command:
vi = interpn(u,v,w,x,z,uu,vv,ww,xx,'spline');
where u,v,w,x are vectors of the same length defining the point with the value z. uu,vv,ww,xx are creaed by the ndgrid command. i got the error message: "wrong number or incorrectly formatted input argument".

thanks for any help
Thanatos