The -Power- button on the left side of the JBL Clip 4 speaker.
When the Powerbutton on your JBL Clip 4 is not working, this means one or more of the following.
Dead Battery. The internal battery is empty needs a recharge for continued speaker use. So use a commercially available USB-C charger for this.
Faulty AC Adapter. The power adapter charger is defective. So replace it with a charger that delivers 5 volts, at 0.5 amps (at least 2.5 watts).
Faulty USB-C Power Port. This input port for charging the speaker may have damage. Indeed this is a common problem on this speaker. So replace this if you have the skills.
Stale Battery. That battery no longer recharges due to old age, repeatedly overcharging, very long storage without recharging, or heavy use. Replace the battery in this case with one that delivers 3.7 volts at 1.1 amp hours (4 watt-hours, roughly).
Dirty Button Contacts. The button panel has sand, dirt, or other debris in it. So this may prevent the Powerbutton from making proper contact when you poke it. But for this, clean the switches by disassembling the speaker. Then spray, blow, or brush out the contact area.
The Buttons Panel Has Broken. The switch panel is defective, and the answer here is to replace it. These parts are often available on eBay.
Damaged Speaker. Your speaker is defective. If you find this, then the best bet is just to get a new speaker.