Power steering is achieved by a hydraulic pump, and hydraulic pumps can draw a fair amount of power when in use (i.e. when you're turning the wheel.) A so-called "PSPS" - power steering pressure switch - tells the engine control computer that the power steering pressure is above a certain level (as when, say, you are turning the wheel) and the control module will increase the idle RPM a bit to compensate for the additional load to help prevent the engine stumbling or stalling under the load.
In other words, it's normal. 400 sounds a bit high, but an increase in RPM is normal.
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