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Pistons are considerably stronger than valves. When they hit the valve usually just tweaks and will no longer close fully. Worst case the head of the valve breaks off and bounces around in the chamber. Neither issue does anything to the other 3 cylinders.
The vast majority of the time it tweaks the valve just enough to create a decent misfire. Lots of pictures out there of bent valves on other Subaru engines, actual failure rate of the fa20 must be really low.
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