Always change the oil first. Lubrication is the most critical thing for maintaining a diesel. CLEAN fuel is second most important.

Connect a test light to #4 cylinder's glow plug wire terminal. Do the pull action and see if the test light lights. If it does, then the pepper shaker's element is okay (the element acts as a fuse by the way). You MUST have continuity in all 4 glowplugs to complete the circuit.

I'd replace all 4 just on principle.

Pay close attention to the routing of the metal grids. If you get one wrong (usually the one between #3 and #4 behind the injection pump) they will touch the block and short out the system. Go ahead, ask me how I know.

Clean the ground connection (the metal braid or heavy gauge wire leading from #1 to the block) very well. It is as critical connection as the power feed leading to #4. Use a file and wire brush to clean the small loops in the interconnecting grids. Don't bother with any type of protectant grease or sealant such as dielectric grease on the loops. They will just cook it off as they get HOT during the glow process.

Do use antiseize on the glowplug threads themselves. You'll find out why when you take the old ones out. When you take the old ones out, if they bind, spray them with a penetrating oil and tighten them back in a few turns. Back them out, spray and back in. WORK them out, don't force them!

When you have all the plugs out, crank the engine over a few times to blow all the loosened crap out. Now is a good time to do a compression test.

I've only seen one pepper shaker element go bad. That was caused when some fool, ahem, didn't pay attention to the routing of the interconnecting grids.