ESSEN, Germany, May 25 (AFP) - The German company Ruhrgas has increased its share in the giant Russian gas company Gazprom to four percent, from 2.5 percent, Ruhrgas announced here on Tuesday. Ruhrgas chief Friedrich Spaeth estimated last week the cost of the transaction at 500 million marks (255.65 million euros, 278 million dollars). Last December, Ruhrgas had acquired 2.5 percent of Gazprom stock for about 660 million dollars, and said it was interested in buying more. Ruhrgas had turnover of 14.988 billion marks (7.66 billion euros) in 1998, 580 million euros less than the previous year. Net profit was up, however, by 74 million marks to 788 million marks (402.90 million euros).  