\[a \in \left( \frac{1}{4};\frac{1}{2} \right):\]
\[a = \frac{1}{4} \Longrightarrow A = 4 + 2 \cdot \frac{1}{4} =\]
\[= 4 + \frac{1}{2} = 4\frac{1}{2} = 4,5.\]
\[a = \frac{1}{2} \Longrightarrow A = 4 + 2 \cdot \frac{1}{2} =\]
\[= 4 + 1 = 5.\]
\[A \in (4,5;5).\]
\[Ответ:A \in (4,5;5).\]