That's not possible. You can't restrict the user to add only 4 numbers. You could use the workaround Anurag and I mentioned, giving an error, maybe even do g_form.clearValue('u_date') to empty the field.
If really restricting the input, you can look at DOM manipulation. Though that's BAD PRACTICE. So not adviced.
If my answer helped you in any way, please then mark it as helpful.
