Emulex Corp. said its net income for the fourth quarter and fiscal year ending July 2 is expected to be somewhat below current projections by investment analysts. Fred B. Cox, president and chief executive of the Costa Mesa computer storage firm, said a more competitive market for computer storage systems and a less successful product mix has caused the company to lower its revenue expectations. In what was apparently a response to that announcement, Emulex stock fell $1.125 Friday to $9.50 per share, down 10.6% in over-the-counter trading. Cox said pretax profits for the fourth quarter are expected to be about $1 million below the company's own original estimates. The firm did not disclose what it had estimated for the period. "While this new near-term outlook is below previous estimates," Cox said, "we still expect to post an improvement over results for the fourth quarter of 1988."