  We show that certain classes of apparently unprotected operators in N=4 SYM_4
do not receive quantum corrections as a consequence of a partial
non-renormalization theorem for the 4-point function of chiral primary
operators. We develop techniques yielding the asymptotic expansion of the
4-point function of CPOs up to order O(\lambda^2) and we perform a detailed OPE
analysis. Our results reveal the existence of new non-renormalized operators of
approximate dimension 6.
