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The prius will drop from S4 to S3 if the coolant temp drops low enough. Or perhaps you weren't in S4 to begin with. AFAIK (and I could be wrong since you have a 2010), but you need to stop for a good 3 seconds with the engine running to get into S4. If you haven't done that, you aren't in S4 and may or may not be able to stop the engine on demand.
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