По определению косинуса из треугольника АВС:
\(\cos A = \frac{{AC}}{{AB}}\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\,\frac{1}{4} = \frac{{AC}}{{4\sqrt {15} }}\,\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\,\,AC = \sqrt {15} \).
По определению косинуса из треугольника AНС:
\(\cos A = \frac{{AH}}{{AC}}\,\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\,\frac{1}{4} = \frac{{AH}}{{\sqrt {15} }}\,\,\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\,\,AH = \frac{{\sqrt {15} }}{4}\).
По теореме Пифагора из треугольника AНС:
\(A{C^2} = A{H^2} + C{H^2}\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\,\,C{H^2} = {\left( {\sqrt {15} } \right)^2} — {\left( {\frac{{\sqrt {15} }}{4}} \right)^2}\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\,\,\,CH = 3,75\).
Ответ: 3,75.