CORRECTED-UPDATE 4-Arconic CEO quits over letter sent in "poor judgment" to Elliott (April 17)

April 17 (Reuters) - Arconic Inc said on Monday Chief Executive Klaus Kleinfeld resigned after the U.S. specialty metals maker found he sent a letter in "poor judgment" to Elliott Management, with whom it is embroiled in a proxy war and which used the chance to again criticize the company's board.