On June 24, 1982, a Boeing 747 operated by British Airways ran afoul of volcanic ash, resulting in the failure (for a relatively brief but probably eternal-feeling period) of all four engines. One of my favorite aviation quotes comes from this event, made by Captain Eric Moody to the passengers: "Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress."
After almost 15 minutes in a controlled gliding descent, the crew coaxed one engine back to life, allowing them to significantly reduce their rate of descent. A few minutes later, a second engine came back on line with partial power, allowing the aircraft to enter a slow climb. Speedbird 9 eventually made a safe emergency landing at Jakarta with three out of four engines running, albeit not smoothly. A few weeks later, a Singapore Airlines Boeing 747 flying through the same airspace also lost three out of four engines for a brief period due to volcanic ash ingestion. Again, successful in-flight restarts resulted in a safe emergency landing.
Consider this: 747s have the ability to dump fuel and reduce their weight to facilitate emergency landings and/or prolonged flight on fewer than all four engines. Crews practice two-engine-out emergencies in simulator training. The engines themselves can take tremendous abuse. The most credible causes of multiple engine failures on a Boeing 747 would be (1) bad fuel (which is why testing is so frequent and rigorous); (2) multiple simultaneous strikes by large birds on multiple engines; or (3) volcanic ash ingestion similar to that experienced by the BA 747. The latter case is the most plausible, which is why the Icelandic volcano Eyjafjallajökull's eruption in 2010 was treated so seriously by aviation authorities.
