Dell (DELL) shares tumbled more than 6% in late morning trading Wednesday after the computer maker late Tuesday missed its Q4 earnings target and guided to lower-than-expected sales in the current quarter. Analysts with Citigroup and Needham & Co. cut their ratings from the equivalents of buy to hold. ISI Group said Dell’s results cast doubt on its ability to grow earnings per share this year. “In the face of flat revenues, declining gross margins