If you found an AirTag, have the serial number and last four of the phone number, can we find the owner?

There is perhaps one hope for the AirTag if the owner has marked it as lost and in that process left some sort of note about how they can be contacted.

In What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you the support document Apple has written has a section called Get information about or disable an AirTag, Find My network accessory or set of AirPods

This section says (and refer to point 3)

If you have found an unknown AirTag, you can follow these steps to see information about it or to disable it: 

1. If you have found an AirTag, hold the top of your iPhone or NFC-capable smartphone near the white side of the AirTag until a notification appears.
2. Tap the notification. This will open a website that provides information about the AirTag, including its serial number and the last four digits of the phone number of the person who registered it. This can help you identify the owner, if you know them. You may want to take a screenshot to make sure you can document the information.
3. If the owner marked the AirTag as lost, you may see a message with information about how to contact the owner.

That's a long-shot, given that the point of an AirTag is for the owner to keep track of it...

But as Axel has said, even if you have this information you might be more comfortable in taking it to a local police station for them to do the contacting.

No. If you found an AirTag, then what you can do is disable it as instructed in your iPhone. After that, you can choose to destroy it, turn it in to the authorities etc.

Axel F.