$P={{P}_{{}^\circ }}+\rho gh\Rightarrow P-{{P}_{{}^\circ }}=\rho gh\Rightarrow \frac{{{P}_{2}}-{{P}_{{}^\circ }}}{{{P}_{1}}-{{P}_{{}^\circ }}}=\frac{{{\rho }_{2}}}{{{\rho }_{1}}}=\frac{0/4{{P}_{{}^\circ }}}{0/2{{P}_{{}^\circ }}}=2$