Good afternoon NormaMG,
Welcome to the Apple Support Communities!
I understand you have questions regarding setting up your Apple Watch Series 4 with your Cellular provider. Which cellular provider are you working with? Different carriers may have different methods for offering the cellular service. For example, Verizon uses the same phone number for the Apple Watch as is used for the iPhone.
Verizon - NumberShare FAQ
Share your smartphone's number with compatible smartwatches, Palm and other eligible 4G LTE connected devices using NumberShare. Use one number to make and take calls, get app notifications and more on all your synced devices.
You will need to ask your cellular provider how their service works and confirm the costs. Make sure your Apple Watch is updated to the latest watchOS and you have updated your carrier settings. Then restart your Apple Watch to refresh it.
Wireless carrier support and features for iPhone in the United States and Canada
Update your carrier settings on your iPhone or iPad
Update your Apple Watch
Take care.