updating apple watch series 4 with a iphone

And please do not try any tricks you think up to get around this. If you manage to get your Apple Watch upgraded to watchOS 6 it will never pair to your iPhone 6 again.

Keep your Apple Watch at watchOS 5 until some time in the future you decide it is time to get a new iPhone. You will be much happier in the long run.

You cannot. You need at least a 6S.