\begin{frame}
  \frametitle{Fourth Normal Form}

  \begin{block}{Fourth Normal Form (4NF)}
    A relation is in \emph{Fourth Normal Form (4NF)} if every MVD 
    \begin{center}
      $A_1, \dots, A_n \mvd B_1,\dots, B_m$
    \end{center}
    is 
    \begin{itemize}
    \item either trivial, or
    \item implied by a key.
    \end{itemize}
  \end{block}
  
  This definition of 4NF is very similar to BCNF  but with a
  focus on implied MVDs (not FDs).
  \pause\medskip
  
  \begin{goal}{}
    Since every FD is also an MVD, 4NF is
    stronger than BCNF.\\
    \remark{%
      That is, if a relation is in 4NF, it is automatically in BCNF.
    }
  \end{goal}
  \remark{%
    However, it is not very common that 4NF is violated, but BCNF is not.
  }
\end{frame}