The Gamin Vivomove 3 is mostly a fitness tracker with some smartwatch capabilities. Unlike the Apple Watch, the Vivomove 3's monochrome display is tiny and barely interact-able i.e. you can't respond to a notification on a Vivomove the way you do with an Apple Watch. Fitness tracking aside, you can't send a text message or email from the Vivomove and neither can you answer or make a call from your watch when you're separated from your phone.
The Series 4 is basically a tiny wrist computer with a 1000 nits color display, a 64-bit processor, ECG sensor, and lots of connectivity (Bluetooth, Wi-Fi 5.0, and LTE if you have a cellar model). It lets your store your flight tickets, pay for food, and ride public transit. It also has complications that update periodically through the day. You could probably get 2-3 days out of your Apple Watch's battery by disabling most of those features, but that would no longer make your Apple Watch an Apple Watch.