Complete the table and find the rule: Input [x]: 1, 2, 4, (missing value) Output [y]: 1, 4, 9, 25

The given values are:
Input $[x]$: 1, 2, 4, (missing value)
Output $[y]$: 1, 4, 9, 25

Step 1: Analyze the outputs.
Notice that the outputs are perfect squares:
$1 = 1^2$
$4 = 2^2$
$9 = 3^2$
$25 = 5^2$

Let $k$ be the base of the square, so $y = k^2$. We can list the corresponding $k$ values:
For $x=1