These JBL Charge 4 hard reset instructions describe how to hard reset this speaker. Hard resetting puts the speaker back into its default state. Plus, it wipes any earlier paired devices from memory. This stops it from pairing to any nearby devices afterwards.
JBL Charge 4 Hard Reset Instructions: Step by Step
1. Turn on the JBL Charge 4
Press and release the Power button. It should light up solid white right away.

Find the Power button, pointed at by the green arrow in the last picture above.
Then, the Power button comes on, as shown next. Here, since we’ve paired the speaker with Bluetooth devices before this, it does not enter Bluetooth discovery mode by itself. We know that because the Bluetooth button remains OFF, rather than blinking white. Note the dark Bluetooth button, to the right of the glowing Power button in the next picture.

2. Wait for the Power Up Sound
The speaker plays a rising electric guitar strum sound when it powers up. Then it is ready for hard resetting once this sound ends.
3. Press the Volume UP and Bluetooth Buttons Together to Continue with JBL Charge 4 Hard Reset Instructions
See the next picture to find the Bluetooth and UP Volume buttons.

The speaker then enters hard reset, as shown in the next picture. Note that both the Power and Bluetooth buttons and the egg shaped ring that surrounds them. All of these light up for a couple seconds during reset. I.e. The ring and both buttons glow gray-white.

Then after a short pause, the JBL Charge 4 turns off. All lights go off, and it makes the power-down guitar sound.
The next picture shows the JBL Charge 4, powered down. All buttons are dark.

4. Check that your JBL Charge 4 Did Hard Reset
Next, power on the speaker again, as we did in step 1 above. In short, press and release the Power button.
Now this time, after hard reset, our speaker instantly comes up in Bluetooth discovery mode, as shown next. We did not have to press the Bluetooth button first. Note that the Bluetooth button blinks to let you know that the speaker has entered Bluetooth discovery mode. When this button blinks or is dark, no connection is active. Indeed, we expect it not to pair to anything afterwards.

5. Done with JBL Charge 4 Hard Reset Instructions !
The the hard reset is now complete. Thus you may now pack it up for sale, or pair it again with your favorite Bluetooth devices.
Now again, the speaker will no longer automatically pair with any previously paired device. So you forget the JBL Charge 4 in the Bluetooth devices section of your source device settings. Once you do that, then manually pair with that device again to refresh the connection info. Then any time that both speaker and that device have Bluetooth running, pairing with automatically happen again.