T-Bill Rates Rise: Interest rates on short-term Treasury securities rose to the highest level in more than four years. The Treasury Department sold $7.2 billion in three-month bills at an average discount rate of 9%, up from 8.69% last week. Another $7.2 billion was sold in six-month bills at an average discount rate of 9.04%, up from 8.76% last week. The rates were the highest since Oct. 29, 1984, when three-month bills sold for 9.38% and since six-month bills averaged 9.07% on Nov. 5, 1984. The latest 9.04% average for six-month bills had been matched on March 18, 1985. The new discount rates understate the actual return to investors--9.34% for three-month bills, with a $10,000 bill selling for $9,772.50, and 9.60% for a six-month bill selling for $9,543.