Putting the "cap" back on a stem I was cleaning up, the cheesy little 4.5 mm cap-screw snapped off with but little torque.
I removed the other three and sure enough, about 3/6" of broken screw still protruded from the stem.
I took my Dremel and a cut-off wheel and cut a slot in the thing down to the level of the stem surface.
Fitted a screwdriver... Out she came. A liberal dose of WD-40 beforehand doesn't hurt.