Have new iPhone6.. Can't backup from iCloud as older iOS than old phone

I believe you are saying your iPhone 4S has a later version of iOS 8 than your iPhone 6 does.

If that is true, do this on your iPhone 6:

Settings > General > Software Update

Then you can restore your iCloud backup from the 4S. (Start that by doing Settings > General > Reset > Erase All Content and Settings after the above software update.)

Go through the initial setup and then update to ios 8.3. You can then return tho the setup again by using Settings > General > Reset > Erase All Content and Settings. During this setup you can restore to your backup.

Go through those initial questions. Set your iPhone up as new. Then do the software update. Then do the Erase All Content and Settings and restore from iCloud.