tangent:

A short will definitely cause the filament to break as it will be overheated.

Check the next glow plug (#3) for a short to ground. If that one is grounded, it will burn out #4.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!