\[\boxed{\text{4.\ 338}}\]
\[\textbf{а)}\ 1\frac{3}{4}\ :3,75 = 4x\ :15\]
\[4x \cdot 3,75 = 15 \cdot 1\frac{3}{4}\]
\[15x = 15 \cdot 1\frac{3}{4}\]
\[x = 15 \cdot 1\frac{3}{4}\ :15\]
\[x = 1\frac{3}{4}.\]
\[\textbf{б)}\ \frac{1}{2}\ :13 = \frac{1}{3}x\ :4\frac{1}{3}\]
\[\frac{1}{3}x \cdot 13 = 4\frac{1}{3} \cdot \frac{1}{2}\]
\[\frac{13}{3}x = \frac{13}{3} \cdot \frac{1}{2}\]
\[x = \frac{13}{3}\ :\frac{13}{3} \cdot \frac{1}{2}\]
\[x = \frac{1}{2}.\]