So if I set up an apple watch 4 or later with cellular on family plan under my phone but dont want to pay for a watch plan. would they still have a phone number I could call them on while on WIFI?

Thanks for posting in the Apple Support Communities lakebum1,

Whether or not Apple Watch has an active cellular plan, it would not have its own dedicated cellular number. Cellular plans for Apple Watch are for data. Features that require data, may be unavailable when the device is out of range of Wi-Fi, without a cellular data plan.

 A cellular plan isn't required to set up an Apple Watch for a family member, but is necessary for some features.

Set up Apple Watch for a family member

We hope this helps!