$\mathrm{P}_{\mathrm{i}}=\mathrm{P} \quad \mathrm{P}_{\mathrm{f}}=4 \mathrm{P}$

By gas equation we know: $-V=\frac{n R T}{P}$

$\therefore \mathrm{PV}^{4 / 3}=\mathrm{constant}$

$\Rightarrow P\left(\frac{n R T}{P}\right)^{\frac{4}{3}}=$ constant

$\Rightarrow \frac{T^{\frac{4}{3}}}{p^{\frac{1}{3}}}=$ constant

$\therefore T_{f}=\left(\frac{P_{t}}{P_{i}}\right)^{\frac{4}{3}-\frac{4}{3}} \times T_{i}=\left(\frac{4 P}{P}\right)^{\frac{1}{4}} \times 300 \mathrm{K}$

$\Rightarrow T_{f}=300 \sqrt{2} \mathrm{K}$