[string]$stringToPad = 1
$padChar = "X"
$paddedLength = 4
$stringToPad = ($padChar * ($paddedLength - $stringToPad.Length)) + $stringToPad

$padChar = "X"
$paddedLength = Y
$stringToPad = ($padChar * ($paddedLength - $stringToPad)) + $stringToPad

For clarity, I have broken two variables out, but this could easily be a one-liner.
Usage:
Replace X with the character you wish to use to pad (e.g. 0 (zero)).
Replace Y with an integer representing the desired TOTAL length of the string.

Left-padding in Powershell

Windows PowerShell