Implement boolean expression with AOI
The result of Step 2 has obvious two 2X2 AOI. One for the left hand side let us call it X and one for right hand side let's call it Y.

\begin{equation}
X = \overline{PS + QS} \; (one AOI22)\\
Y = \overline{R\overline{S} + \overline{S}T} \; (one AOI22)
\end{equation}

Also you can implement the inverter using AOI22 as below:
\begin{equation}
\overline{(S.1 + 0.0)} = \overline{S} \; (one AOI22)
\end{equation}

What you need is 
\begin{equation}
\overline{X} + \overline{Y} 
\end{equation}

which you can perform by one 2x2 AOI gate as: 

\begin{equation}
\overline{(X.Y + 0.0)} = \overline{X} + \overline{Y} \; (one AOI22)
\end{equation}

This makes 4 AOI22 for the result of Step 2.