I have removed dozens of broken plugs from 4.6/5.4. The best way is on a cold engine, try to loosen the plug 1/8th turn, then fill the spark plug well with carb spray and them sit for 30 minutes and try to remove them. On the ford engine the plugs can break 3 different ways, you need a removal kit to get the broken plug out. We service the county trucks and a fleet of 25 commercial ford busses. Thread repair inserts also are common on these engines. The best way I found is to let them soak on a cold engine and work the plug carefully trying not break them off.