Correct way to handle .then continuation

Correct way to handle .then continuation

I have a .then continuation that should either call another asynchronous function or not do anything. What is the correct way to write the function? Here it is:

    function loadCategories(shortDescription, categoryId, index) {
        return categoryService.loadChildCategories(shortDescription, categoryId, index)
            .then(function (result) {
                return categoryService.prefetchChildren(shortDescription, result, index + 1);
            });
    }

I want to call prefetchChildren only if index < 4. Question is what do I return if index >= 4 to make the function chainable with .then?