How to do transfer data via iTunes from iPhone 4S to 6S
You will need iTunes 12, which can only be run on newer versions of OS X. From the Mac App Store, download the free upgrade of OS X 10.11 El Capitan. Then, update iTunes. From there, you can just plug in your 4s, back it up, and then plug in the 6s and restore from that backup. Make sure you choose Encrypt iPhone Backup so you won't have to sign in to every app again after the restore.