This should be working. There are a couple of things, the first is that the actual code because there’s an error in it, it should be
add_filter('acf/validate_value', 'my_acf_validate_value', 10, 4);
function my_acf_validate_value( $valid, $value, $field, $input ){
return "is that working?";
}
and that error will cause a PHP error that should cause it to fail.
The other thing that may have something to do with it is, where are you adding the above code? Is it in functions.php?
