Believing its stock to be substantially undervalued following the stock market collapse Oct. 19, MAI Basic Four has declared a special dividend of $1 per share, payable Jan. 5, 1988, to shareholders of record Dec. 24. The Tustin-based computer company, which has not paid a regular dividend, had 14.6 million common shares outstanding as of Sept. 30. According to Fred D. Anderson, MAI vice president and chief financial officer, the company did not want to buy back stock as dozens of companies have in the wake of the market crash because that doesn't provide any long-range benefit for the shareholder. Anderson said the dividend is an "indication of confidence in the long-term future of the company." And the announcement has already had a positive impact, he said: the company's stock closed Wednesday at $14, up $1.25 for the day, in New York Stock Exchange trading.