4mm=.1575", a #21 drill is .1590". I expect you could run a #21 into the existing hole and open it up. But the drill will be bending since the lip in the vent field is there so there is some risk of breaking the drill off in the hole. Maybe use each drill from a #28 to a #21 so you aren't cutting much at a time.
Or make the quill on the spot using the hole as the straw. Can't imagine you're going to be firing volley with it.
“If you're not a liberal at 20, you have no heart; if you're not a conservative at 40, you have no brain.”--Winston Churchill
Last edited by GGaskill; 03-22-2017 at 06:43 PM.