JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <input name="reg" id="reg" type="text">
</form>

JavaScript

$reg = $( '#reg' );
$reg.blur( function() {
    while( this.value.length < 4 )
    {
        this.value = '0' + this.value;
    }
} );