In order to set up the Apple Watch 4, you will need an iPhone with the iOS Watch app, as well as a cellular plan with a cell provider that supports Apple Watch (such as T-Mobile, Verizon, ATT or Sprint.) As you note, the watch will sync up with the number of your cellular plan. The watch will share data from your data plan. You will also pay a fee to have the watch on your cellular plan (usually around 10.00 with autopay.) Once that is all set up, you can leave the phone at home and rely on the watch for functions like calls, texts, emails, health tracking and such.