For given current distribution, each infinite length wire produces magnetic field ‘B’ at origin then resultant magnetic field at origin ‘O’ is:

Due to conductor P, B at 0, $B_1=\frac{{\mu }_{0}I}{2\pi r}\widehat{j}$
Due to conductor Q, B at 0, $B_2=\frac{{\mu }_{0}I}{2\pi r}.\widehat{j}$  
$B_{Net}$at O, due to P & Q $=2.\frac{{\mu }_{0}I}{2\pi r}.\widehat{j}=2B\widehat{j}$
due to conductor R, B at 0, $B_3=\frac{{\mu }_{0}I}{2\pi r}.\widehat{i}$ 
due to conductor S, B at 0, $B_4=\frac{{\mu }_{0}I}{2\pi r}.\widehat{i}$ 
$\therefore B_{Net}$ at 0 due to $B_3\&B_4=2\frac{{\mu }_{0}I}{2\pi r}\widehat{i}=2B\widehat{i}$ 

$\therefore B_{Net}$ at 0, due P, Q, R, S $=2\sqrt{2}\mathrm{B}$