To find hypotenuse $ c $ use formula:
$$ \cos \left( \beta \right) = \dfrac{ a }{ c } $$After substituting $ \beta = 50^o $ and $ a = 4 $ we have:
$$ \cos( 50^o ) = \dfrac{ 4 }{ c } $$ $$ 0.6428 = \dfrac{ 4 }{ c } $$ $$ c = \dfrac{ 4 }{ 0.6428 } $$ $$ c = 6.2229 $$