4. A water pool has 5 inlet and outlet water pipes numbered (1), (2), ..., (5). It is known that the time it takes to fill the pool when certain pipes are opened is as shown in the table below. If all 5 pipes are opened simultaneously, ( ) hours will be needed to fill the pool.
(A.) $\frac{1}{5}$
(B) $\frac{1}{2}$
(C.) 1 (D) 2
\begin{tabular}{c|c|c|c|c|c} 
Pipe Number & (1)(2) & (2)(3) & (3)(4) & (4)(5) & (5)(1) \\
\hline Time (hours) & 2 & 4 & 7 & 14 & 28 \\
\hline
\end{tabular}
4. (D).

Consider the sum of the fractions of the pool that can be filled in one hour by doubling the rate of all the pipes:
$$
\frac{1}{2}+\frac{1}{4}+\frac{1}{7}+\frac{1}{14}+\frac{1}{28}=1 .
$$

This shows that the pool is exactly filled because the combined rate of the 5 pipes doubled for one hour