MGM/UA stockholders of record on March 13 will be entitled to buy one share of convertible preferred stock for each four shares of common they own. The price is $18 a share, the same as controlling Kirk Kerkorian paid in January . . . The Shidler Group and Los Angeles-based CalMat Co. have agreed to terminate negotiations concerning the purchase by the Shidler Group of certain income-producing properties owned by CalMat . . . South Korean auto maker Hyundai, facing a decline in sales, said it will offer its first U.S. rebates of up to $1,000 on selected 1988 and 1989 Excel models.