I have my doubts regarding the feasibility of SDBs to be shot down in-flight by any air defence system (being it S-400, Pantsir, Tor, etc...). SDBs (Small diameter bombs) like the name indicates are very small bombs which means they have a very small RCS, which should make them "stealth" or "almost stealth".

Besides I have yet to see any defence system to be effective at shooting down any smart weapons (Paveway or JDAMs for example) specially outside a "non-controlled" environment (i.e. a test environment) or resuming in a real combat scenario/environment.
Anyway even if I'm proven to be wrong with my statements above, one can also use a F-35 equipped with JSOW (which have a similar range of a SDB and it's truly a stealth weapon) to engage those S-400 (or any other air defence system for that matter).