constant selection on restricted key figure

hi gurus:

I m creating a bex query with 4 restricted key figures, I need these 4 RKF in columns with "constant selection" checked but when I mark this option for all this 4 RKF the result of the query is an error, but if I just mark 3 RFK the query Works fine....is there a limit using more tan 3 RFK with this mark?

or do I missing some setting?

regards

mah