It’s a million dollar question: can you downgrade watchOS 4 Beta to watchOS 3 public release?  You can't downgrade watchOS 4 Beta to watchOS 3 unless you’re willing to go to Apple itself.

Once you update to a particular firmware with an Apple watch, you can’t downgrade back to the public release by yourself. There’s no firmware file you can download to roll things back, and there’s no cable that lets you interface with your Apple Watch with a PC or Mac.

If you are willing to take your Apple Watch to Apple itself, then can you roll back to watchOS 3, but that requires you to send off your Apple Watch for a few days. Apple authorized Service Providers as well as Apple Retail Stores can't carry out this service.

It’s a hassle, but nonetheless a solution every user should look given you have Apple’s official presence in your region. Currently, this service is available only in a few countries such as USA, UK, France, Canada, Germany, Italy, Japan, Spain, Switzerland, Australia. 

Make sure to check out these points:

Step 1. Head over to Apple Support page.

Step 2. Next up, you need to select “Apple Watch” as shown in the screenshot below.

Step 3. Now, you have to select “Send in For Service.”

Step 4. Click on Setup & Usage.

Step 5. Next, you have to select Updating my Apple Watch OS.

Step 6. Tap on Talk to Apple Support Now.

Step 7. Up next, you will have to enter the serial number. (On your Apple Watch, go to Settings → General  → About → Scroll down and check out the serial number.)

Now, you will need to ship your smartwatch to Apple for Service. It will take minimum 3 business days to downgrade your wearable device to the previous version.