The Russians provided a very capable air defense system to Syria.
It did nothing to stop an American attack on a Syrian airbase; despite the weapons fired at Syria being the decidedly unstealthy Tomahawk land-attack missile. The only "stealth" this missile has is flying nape of the earth, an envelope the S-400 system is supposed to cover well.
There's really only two possibilities.
One: the system isn't as capable as has been feared or advertised by the Russians.
Two: the Russian operators of the system popped some popcorn and watched those TLAMs come into their area of control and let them hit unopposed.
Either way, that makes the S-400 system useless to Syria.
If the second option is true, and it seems more likely, we need to ask why Russia let the US attack one of their allies with impunity.