Zoom H4n Pro Operation Manual page 70

This function automatically sets the level for recording.
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When REC LEVEL AUTO is used, the unit automatically
sets recording levels using signals present during recording
standby. If the volume surpasses 6 dB during recording, the
input level is automatically adjusted with the new level shown
on the screen for two seconds.
Current input level
Stereo mode 4CH mode
• You cannot use this function together with AUTO REC.
• You cannot make this setting during recording and playback.