Jquery validator regular expression for first four character & after 4 will be number

Jquery validator regular expression for first four character & after 4 will be number


How to write  folllwing regular expression in jquery validator add method

4 chars + 7 digits
 ValidationExpression="^[^\s]{4}\d{7}$"

I want to place above  regular expression in following method

jQuery.validator.addMethod("lettersonly", function(value, element) {
    return this.optional(element) || /^[a-z ]+$/i.test(value);
}, "");