Four Fours/Lemmata/One Four/2 Thirds/Solutions/1

Puzzle: One Four: $\dfrac 2 3$

Using exactly $1$ instance of the number $4$, the task is to write an expression for $\dfrac 2 3$, using whatever arithmetical operations you consider necessary.

Solution

$\dfrac 2 3 = \sqrt {. \dot 4}$