When is a plug not a plug?
 

When it's only half a plug. Or in this case half of a S4 radiator drain plug.

https://cimg7.ibsrv.net/gimg/rennlis...c0f2294fbf.jpg

Any great ideas how to extract the other half from the radiator?

I'd say use a left hand drill bit and an easy out.

take a junker screw driver ,
heat it with a torch,
press into the remaining plug till it melts a slot,
remove and let cool,
then insert the tool and unscrew the plug.

NOTE buy 2 drain plugs and 2 bleed ports for the top of the radiator.
screw the new plugs till snug,
use a smear of Dow Corning 111 on the O rings. keep the extra parts as spares

Ive had success using a hot pick similar to Stans method.......good luck

The hot driver was my first thought too. Just concerned about melting radiator threads.

If you pick the right size screwdriver, with it just hot enough to sink into the plug and no more, you won't have a problem.

+1 on Stan's method. not much torque required to remove the 1/2 plug, so I used a long thin screwdriver. Works great.

Follow up. Warmed up an old Philips head screw driver with a lighter for about 40sec and then pushed it into what was left of the plug firmly for 15sec.

Left an almost "factory" impression of the Philips driver and easily was removed.

Thanks!