For a positive real number $x$, the least value of $x+4/x$ is:

By AM ≥ GM applied to the positive numbers $x$ and $4/x,(x+4/x)/2$ ≥ √$(x\cdot 4/x)=2$, so $x+4/x$ ≥ 4, with equality when $x=4/x$, i.e. $x=2$. (2 is the geometric mean itself — and also the value of $x$ at the minimum — not the least value of the expression.) Sample values: $x=1$ gives 5, $x=2$ gives 4, $x=4$ gives 5.