Series 4 comes with Bluetooth 5 as oppose to Bluetooth 4.2 on Series 3. So as long as you're using it with the latest iPhones that come equipped with Bluetooth 5, the connectivity range should be more.
I tested an iPhone XS Max paired with an Apple Watch Series 4 (both Bluetooth 5) against an
iPhone SE paired with an Apple Watch Series 3 (both Bluetooth 4.2) and got the following results.
1) Range was exactly the same
2) Faster data transfer between phone and watch with the Bluetooth 5 devices