"No country would be as threatened by China's acquisition of the S-400 as Taiwan. As Defense News noted earlier this year, "At present, China's land-based mobile air defense missile systems, HQ-9 and S-300 can reach only a small sliver of northwestern Taiwan….However, with the planned purchase of the 400-kilometer-range Russian S-400 surface-to-air missile (SAM) system, China will for the first time have complete air defense coverage of Taiwan."
Discussing the implications of this for Taiwan's defense, York Chen, a former member of Taiwan's National Security Council, told Defense News: "When S-400s work together with Chinese land- and sea-based fighters, the Chinese will have more confidence in sustaining airspace dominance over the Taiwan theater, thus depriving any organized resistance by the Taiwan Air Force and deterring the American intervention."
http://thediplomat.com/2014/08/china-eyes-russias-s-400-taiwan-seeks-new-air-defense-system/