Apple Watch 4

Yes you may. It is not necessary to activate the cellular portion if you don't wish. The only difference is the cellular watch is more expensive, if you do not intend to activate it. But it is there if you want to later. Understand that most cellular carriers charge a monthly fee for that activation. AT&T charges $10/month. Also, not knowing where you are at, remember you need to purchase the watch in the country that you will use it in, and not all carriers can activate the cellular on the watch. See this, Apple Watch - Carriers - Apple

Yes, it is completely optional to activate and configure the cellular function. Until such time as you choose to add cellular the watch will behave exactly like the GPS-only version.