Are S-400's really there?
An important question is whether Putin would really put S400 missiles into Syria. He doesn't need them to protect his aircraft from Turkey or anyone else right now. He is arming the fighter-bombers with air-to-air and he has a handful of air superiority fighters there, it is claimed running combat air support. That should be enough to deter future Turkey adventure.
This seems like a lot of bluster for the suburbs of Moscow and environs. Face it - as stated in some earlier posts, Israel and Russia have a unique relationship, but it is a relationship. If Israel would be angry about S300's, they would go "ballistic" about S400's. Plus they would have to be manned by Russians, which exposes their personnel significantly. Plus this would unnecessarily expose their radars and other electronic signatures to several surveilling NATO countries and Israel. Not that this intel hasn't already been obtained by these countries, but still.
Remember, S400's are an AD/AA tool, meant to stop bombers and fighter bombers from dashing into Russia's heartland to drop nuclear weapons. They are really more of a strategic than tactical asset. Sticking them down in Syria seems incredible overkill with substantial risks. The Russians have other effective battlefield anti-air missiles they could employ for the tactical threats.
Finally, nobody has yet spotted the key 92N6E "Grave Stone" fire control radar that manages the total threat environment for the S400. True, some ancillary radar assets have been seen and reported, but not the keystone of the S400 radar system, as far as I have heard. I suspect there aren't any S400's in Syria.