Zoom H4n Pro Operation Manual page 23

These keys allow you to adjust the levels of recording input and output volume.
(All the modes are the same.)
Recording level
The recording levels for INPUT 1 and 2 can be adjusted
Press the INPUT
MIC, 1 or 2 button
to select an input
source to adjust its
recording level.
The volume from the built-in speaker and output jacks
1–100
+: Increase sensitivity
(amplify)
–: Decrease sensitivity
(attenuate)
0–100
+: Increase volume
–: Decrease volume
Volume
022