The latest Android update has caused the Clock app on some phones to stop sounding alarms. This affects all Pixel models, and not just the 4. To fix, go into "Settings", Apps & notifications", "See all Apps", scroll down and choose "Clock", choose ""Storage and cache", choose "Clear storage". A window will pop up that reads: "Delete app data? All this app's data will be deleted permanently. This includes all files, settings, accounts, databases, etc." Touch "OK".
While this will fix your alarms, it will also erase all of your alarms, so you will have to relaunch Clock to add your alarms back, but it does work. Just to make sure it worked, set an alarm for a few minutes from now, then wait to confirm that it is working again.
Disclaimer: The Product Ambassadors are Sprint employees from many different parts of the company that love technology. They volunteer to test out all sorts of Sprint devices and offer opinions freely to the Community. Each Product Ambassador shares their own opinions of these devices, therefore the information in this post does not necessarily reflect the opinions of Sprint. The PA's do not represent the company in an official way, and should not be expected to respond to Community members in an official capacity. #sprintemployee