* PBOC may tighten cash supply to address inflation risks
* Corning jumps, Samsung unit may become largest shareholder
* Futures off: Dow 74 pts, S&P 10 pts, Nasdaq 21 pts
NEW YORK, Oct 23 (Reuters) - U.S. stock index futures fell on Wednesday, following four days of record highs on the S&P 500, amid concern over tightening financial conditions in China and weakness in European banks.
* China's primary short-term money rates rose after a policy adviser to the People's Bank of China said the PBOC may tighten its cash supply to address inflation risks, a move that could hurt growth in the world's second-largest economy.
* The European Central Bank said it will put top euro zone banks through rigorous tests next year to build confidence in the sector. However, some analysts say if the review reveals unexpectedly large problems, it could backfire by undermining the confidence it aims to bolster.
* Shares of European banks dropped 2 percent and the Select Sector Financial SPDR fell 0.6 percent in premarket trading on Wall Street.
* S&P 500 futures fell 10 points and were below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration on the contract. Dow Jones industrial average futures fell 74 points and Nasdaq 100 futures lost 21 points.
* A unit of Samsung Electronics could become the biggest shareholder of Corning Inc, the maker of scratch-resistant Gorilla Glass used in many mobile gadgets. Corning shares jumped 22 percent in premarket trading.
* According to Thomson Reuters data, of the 128 companies in the S&P 500 that have reported earnings, 63.3 percent have topped analysts' expectations, in line with the beat rate since 1994 but below the 66 percent rate over the past four quarters.
* S&P 500 companies reporting earnings Wednesday include Boeing, AT&T Inc, Caterpillar and General Dynamics.