I have a Series 4 GPS only model. I always have my iPhone with me, or nearby. I can do Everything my Wife's cellular Apple Watch can do.
Actually, I do a little bit better, because we live in a weak cell signal area, and my iPhone picks up the weaker signals better than the Apple Watch.
I just cannot leave my iPhone behind, and that only happens by mistake, and very rarely at that, as it is generally always in my pocket, unless I'm using it, or it is charging.
NOTE: Even the Cellular mode uses the iPhone if it is in range, as using the Apple Watch cellular radio cost more battery power and shortens the time between charges. leaving the phone at home, while you go off to work, shortens the Apple Watch day significantly, and it will need a charge sooner than when you go to bed at night
Apple Watch battery is rated for 18 Hours between charges (the reality varies depending on how new the battery is, how often the Apple Watch is used to make phone calls, how many hours are spent exercising such that the GPS is tracking your movement, the amount of time spent listening music stored on the Apple Watch vs streaming music over the cellular connection, etc...