To find radius $ r $ use formula:
$$ V = \dfrac{ r ^{ 2 } \cdot h \cdot \pi}{ 3 } $$After substituting $ V = 27 $ and $ h = 4 $ we have:
$$ 27 = \dfrac{ r ^{ 2 } \cdot 4 \cdot \pi}{ 3 } $$$$ 27 \cdot 3 = r ^{ 2 } \cdot 4 \cdot \pi $$$$ 81 = 4 \cdot r ^{ 2 } \cdot \pi $$$$ r ^{ 2 } = \dfrac{ 81}{ 4 \, \pi } $$$$ r ^{ 2 } = 25.7831 $$$$ r = \sqrt{ 25.7831 } $$$$ r = 5.0777 $$