To find side $ a $ use formula:
$$ r = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$After substituting $r = 4\, \text{cm}$ we have:
$$ 4\, \text{cm} = \dfrac{ \sqrt{ 3 } \cdot a }{ 2 } $$ $$ \sqrt{ 3 } \cdot a = 4\, \text{cm} \cdot 2 $$ $$ \sqrt{ 3 } \cdot a = 8\, \text{cm} $$ $$ a = \dfrac{ 8\, \text{cm} }{ \sqrt{ 3 } } $$ $$ a = \frac{ 8 \sqrt{ 3}}{ 3 }\, \text{cm} $$