H4n Pro is smart so the adjustments you need happen automatically, by default.
Operation Manual, Page 045, under 4CH Mode, on the right side of the page about ½ ways down it reads: “Connecting an external microphone sets input automatically.” The graphic is clear.
Only the On-board Mic(s) OR the Ext. Mic (backside) can be used at one time when recording, but not both simultaneously. You need to choose one, either On-board Mic(s) or Ext. Mic.
You can use the XLR 1 & XLR 2 simultaneously with your Mic selection.
When the Ext. Mic port is plugged in to something, the H4n Pro will ignore the On-board Mic(s). The Mic(s) is temporarily disabled or turned off, automatically, because the unit recognizes the Ext. Mic is “plugged in.”
To Monitor and Adjust Record Levels of the Ext. Mic, just press the MIC button, underneath the INPUT button on the front of the Unit.
To Monitor and Adjust Record Levels of XLR 1 & XLR 2, just press the 1 or 2 button on the front. If 1 & 2 are “Linked”, just press 1.