South Korea's state-run Korea Gas Corp. (KOGAS) said Wednesday it has found an additional reserve of natural gas in waters off Africa's Mozambique.   The latest discovery was made at the Area 4 offshore well of Mozambique, in which KOGAS has a 10 percent stake.  The newly discovered reservoir is estimated to hold 7.5 trillion cubic feet (Tcf) or about 170 million tons of natural gas. With the earlier discovery of 510 million tons, future production at the Mozambique well will yield about 68 million tons for South Korea, which is enough to supply the country's total consumption for nearly two years, according to KOGAS.