woocommerce_short_description limit

Hi i have this code, but it's doesn't work
I want to limit user's to use only 4 numbers on short description. any help????

add_filter( 'woocommerce_short_description', 'prefix_filter_woocommerce_short_description' );
function prefix_filter_woocommerce_short_description( $post_post_excerpt ) {
{ ?>
<script>

function Onlyletters(e, t) {
try {
if (window.event) {
var charCode = window.event.keyCode;
} else if (e) {
var charCode = e.which;
} else {
return true;
}
if (
(charCode > 64 && charCode < 91)
(charCode > 96 && charCode < 123)
(charCode > 191 && charCode <= 255)
){
return true;
} else {
return false;
}
} catch (err) {
alert(err.Description);
}
}

</script>
<?php }
}