\(\sin \theta_{2}=\frac{L / 2}{\sqrt{(L / 2)^{2}+(L / 2)^{2}}}=\frac{1}{2(\sqrt{1 / 4+1 / 16})}=\frac{2}{\sqrt{5}}\)
\(\Rightarrow B=\frac{\mu_{0} i}{{4 \pi \frac{L}{4}}}\left(\frac{2 \times 4}{2 \sqrt{5}}\right) B=\frac{4 \mu_{0} i}{\sqrt{5} \pi L}\)