Step 1: Write the given data.
Initial pressure,
\[
P_1=4\ \text{atm}.
\]
For a monoatomic gas,
\[
\gamma=\frac{5}{3}.
\]
Initial temperature,
\[
T_1=27^\circ\text{C}=300\ \text{K}.
\]
Final temperature,
\[
T_2=327^\circ\text{C}=600\ \text{K}.
\]
Therefore,
\[
\frac{T_2}{T_1}
=
\frac{600}{300}
=
2.
\]
Step 2: Use the adiabatic relation between temperature and pressure.
For an adiabatic process,
\[
TV^{\gamma-1}=\text{constant}
\]
and
\[
PV^\gamma=\text{constant}.
\]
Eliminating \(V\), we obtain
\[
T^\gamma P^{\,1-\gamma}
=
\text{constant}.
\]
Hence,
\[
\left(\frac{T_2}{T_1}\right)^\gamma
=
\left(\frac{P_2}{P_1}\right)^{\gamma-1}.
\]
Therefore,
\[
\frac{P_2}{P_1}
=
\left(\frac{T_2}{T_1}\right)^{\frac{\gamma}{\gamma-1}}.
\]
Step 3: Substitute the values.
Since
\[
\gamma=\frac{5}{3},
\]
\[
\gamma-1=\frac{2}{3}.
\]
Thus,
\[
\frac{\gamma}{\gamma-1}
=
\frac{\frac{5}{3}}{\frac{2}{3}}
=
\frac{5}{2}.
\]
Hence,
\[
\frac{P_2}{P_1}
=
2^{\frac{5}{2}}.
\]
Therefore,
\[
P_2
=
4\times 2^{\frac{5}{2}}.
\]
Since
\[
4=2^2,
\]
\[
P_2
=
2^2\times 2^{\frac{5}{2}}.
\]
\[
P_2
=
2^{\frac{9}{2}}\ \text{atm}.
\]
Step 4: Final conclusion.
Hence, the final pressure of the gas is
\[
\boxed{2^{\frac{9}{2}}\ \text{atm}}
\]
Therefore, the correct option is
\[
\boxed{(4)}
\]