  The content of the OPE of two 1/2 BPS operators in N=4 SCFT$_4$ is given by
their superspace three-point functions with a third, a priori long operator.
For certain 1/2 BPS short superfields these three-point functions are uniquely
determined by superconformal invariance. We focus on the cases where the
leading ($\theta=0$) components lie in the tensor products
$[0,m,0]\otimes[0,n,0]$ and $[m,0,0]\otimes[0,0,n]$ of SU(4).
  We show that the shortness conditions at the first two points imply selection
rules for the supermultiplet at the third point. Our main result is the
identification of all possible protected operators in such OPE's. Among them we
find not only BPS short multiplets, but also series of special long multiplets
which satisfy current-like conservation conditions in superspace.
