Apple Watch Series 4 Compatibility

Short answer- no.

It depends if the prior owner has or hasn’t updated to watchOS 6. If it still has watchOS 5, you can pair. Check with the owner to know for sure what version of watchOS it has installed and be sure they follow the steps outlined here: https://support.apple.com/en-us/HT208855

If you get the Series 4 new in an unopened box, then it will have watchOS 5 and it will pair with the iPhone 6.

If it is a used Apple Watch, then it depends on whether the previous owner upgraded it to watchOS 6, which would not pair with an iPhone 6.

Also if used, that the previous owner followed the following instructions

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else

https://support.apple.com/en-us/HT208855

It depends on the Watch OS

OS 5 yes, OS6 no