Functions $f$ and $g$ defined on the set of real numbers satisfy the equalities
$$\begin{aligned}
& ( f + g ) ( x ) = x ^ { 2 } \\
& ( f - g ) ( 2 x ) = x
\end{aligned}$$
Accordingly, what is the product $f ( 4 ) \cdot g ( 4 )$?\\
A) 45\\
B) 51\\
C) 54\\
D) 60\\
E) 63