Y a-t-il un moyen de redémarrer un Pixel 4a avec un bouton d'alimentation cassé ?

According to Addictive Tips, you can do this on a Pixel phone in the method you started on:

Press volume up and/or down (varies by manufacturer) and plug your device into your computer. Use the following as an outline.

Nexus and Pixel Devices: Press and hold the volume down and plug in the device. When the Google splash screen appears, release it.

If your device does not go into Bootloader using the above method, drop a comment with your device name and model and we’ll try to figure out how to work that one out.

Once the phone is in bootloader mode, run the following command in ADB (must be connected to a PC);

fastboot continue

It will immediately boot your phone.

Please note the error from the provided site noticed by @Robert in the comments below: ADB commands are different than fastboot commands. When the device is in fastboot mode, you use fastboot rather than ADB when targeting the device. You can obtain the drivers necessary from Google or somewhere like XDA.