How to use the F4 key Open the program that uses the key and press F4. At first only the FnLk, Mute, and Volume Up/Down, or F- keys worked while nothing from Microphone Mute (at F4) to Open Start Menu (at F12) worked, except as a Fn key. These voltage signals are further fed into R-C filter or HIGH PASS filter (R2 and C1), to eliminate the noise from the sound. Fn+F4 – Microphone Mute Silences the on-board microphone so it cannot record audio. Step 3: Right-click on the Microphone entry and then click Disable device option. Select the Device properties option under Input.