  We consider holographic CFTs and study their large $N$ expansion. We use
Polyakov-Mellin bootstrap to extract the CFT data of all operators, including
scalars, till $O(1/N^4)$. We add a contact term in Mellin space, which
corresponds to an effective $\phi^4$ theory in AdS and leads to anomalous
dimensions for scalars at $O(1/N^2)$. Using this we fix $O(1/N^4)$ anomalous
dimensions for double trace operators finding perfect agreement with
\cite{loopal} (for $\Delta_{\phi}=2$). Our approach generalizes this to any
dimensions and any value of conformal dimensions of external scalar field. In
the second part of the paper, we compute the loop amplitude in AdS which
corresponds to non-planar correlators of in CFT. More precisely, using CFT data
at $O(1/N^4)$ we fix the AdS bubble diagram and the triangle diagram for the
general case.
