The actual capabilities of the F-35, F-22 and S400 are unknown, so we have to work with the published figures. The F-22 Radar Cross Section (RCS) is the smallest and least detectable of any aircraft currently manufactured or under design anywhere in the world.
The F-35’s published RCS is slightly higher than that of the F-22. However, comments made by many in the military lead most people to understand that the F-35 RCS is actually slightly lower than that of the F-22.
We can speculate, but then, all we have are speculations. What we know is that there is no aircraft manufactured, or under design that can match the stealth of either the F-22 or F-35. So if neither of these planes can take out the S400, then there are no planes that can take out the S400.
If the S400 lives up to its published data, it can detect the RCS of an F-35 sufficiently well to track the F-35 at roughly 55KM. The SDB II (small diameter bomb) carried by the F-35 has a range of roughly 72KM. If these numbers are relatively accurate, then the F-35 can certainly launch an SDB II or two well before it is detected by the S400 radar.
The remaining question is whether or not the F-35 can turn around and escape before the S400 can detect and kill it. The rear aspect of the F-35 has a larger RCS than the frontal aspect so it’s quite likely that the F-35 (or F-22 for that matter) will be at risk.
Many in the military are confident that with the proper training and tactics, the F-35 will prevail against the S400.
Here is an article published by ‘The Business Insider” covering the issue: http://www.businessinsider.com/what-...-syria-2016-10
Aircraft 101 has an excellent primer discussing radar and stealth technologies here: https://basicsaboutaerodynamicsandav...ountermeasure/