In this talk I will present a recent work on superconformal block expansion of four half-BPS operators in free N=4 SYM. The free theory plays an important role in the AdS/CFT correspondence: the disconnected Wick diagrams are dual to the disconnected Witten diagrams, while the connected Wick diagrams are dual, together with the first interacting contribution, to the tree-level Witten diagrams. Moreover, as the protected part of the supergravity correlator is entirely due to the free theory, the results obtained for it are also valid in the interacting theory, as long as one is able to disentagle the protected part from the unprotected one. Slides can be found here.