how to use cellular on apple watch 4 sharir same phone #

If you have a cellular Apple Watch _AND_ you have a cellular carrier for your iPhone that supports the Apple Watch, _AND_ you have paid your carrier to add your cellular Apple Watch to your iPhone plan, then you can use your Apple Watch without having your iPhone nearby.

Here is a list of cellular carrier

https://support.apple.com/en-us/HT205547

Please read this web page

https://support.apple.com/en-us/HT205547

Make sure you read the footnotes.

When you same in sleep mode, do you mean it is powered up, but the screen is dark. Or do you mean powering off your iPhone. There is a difference.

Some features on an Apple Watch need the iPhone, such as SMS messaging (iMessage does not need the iPhone). But for the most part the cellular Apple Watch will function without having your iPhone nearby.

Now a GPS only Apple Watch will also do everything the Cellular model will do as long as the iPhone is nearby.

Bob is correct.

I have the cellular version and I can leave my iPhone in the house while I go for a walk or leave the house. you will need a separate plan with your carrier. I have AT&T and they charge me $10/month plus applicable taxes.