Determine which integral is being approximated with the Riemann sum $\displaystyle\sum _ { k = 1 } ^ { n } \frac { 5 } { n } \sqrt { 4 + \frac { 5 k } { n } }$. Then, calculate the value of $\lim\limits_ { n \rightarrow \infty } \displaystyle\sum _ { k = 1 } ^ { n } \frac { 5 } { n } \sqrt { 4 + \frac { 5 k } { n } }$.