During the ASHA flight CYWG - CYZD, while cruising at FL210, all four engines died suddenly. ATC informed, FL090 requested. Flying manually I stabilized the descent and moved to the FE panel. Engine 1 turned alive after many attempts. Being alone in the cockpit, I switched on A/P and reduced the descent rate. While still descending, I tried to restart the engine No. 4, with success - again after a long time and many attempts. At FL090, flying on 1 and 4, after resetting all fuel valves and pumps and other systems, engines 2 and 3 came back again. I let them warm up, then climbed back to FL210 and to the destination with no issues.
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Real situation:
After switching to the external view, I noticed symptoms of beginning crash to desktop, like the hard disk working hard without reason, no FS gauges response after switching back to cockpit view etc. I tried to save the flight immediatelly, with success. After that, the FS fatal error message has appeared. After restarting the FS and after reloading the flight, aircraft systems were only partially initialised and the simulation was in an undefined state of some kind. The rest of the story is above.