WASHINGTON (AP) _ President Reagan appointed four top business executives Wednesday to a commission charged with finding ''a clear and comprehensive view'' of what triggered the massive drop in the stock market last month.

In a brief meeting in the Oval Office, Reagan talked with members of the group, headed by former Sen. Nicholas Brady, R-N.J.

When a reporter asked Reagan if now is the time to buy stocks, he said, ''One thing I'm not going to do in this job is give advice on the market.''

Named by Reagan to the Presidential Task Force on Market Mechanisms were James C. Cotting, chairman and chief executive officer of Navistar International Corp. of Chicago; Robert G. Kirby, chairman, Capital Guardian Trust Co., Los Angeles; Howard M. Stein, chairman and chief executive officer of Dreyfus Corp., New York, and John Opel, chairman of the executive committee of the International Business Machines Corp., New York.

Robert Glauber, a professor at the Harvard Business School, was named executive director.

White House spokesman Marlin Fitzwater said the president was directing the group to ''assemble the facts needed to give a clear and comprehensive view of what happened to the securities market in October.''

The group was also asked, Fitzwater said, to ''assess the condition of the market and explore and evaluate any changes that need to be made to assure the smooth functioning of these securities markets and to maintain investor confidence.''

He said the task force would report to the president, Treasury Secretary James A. Baker III and the Federal Reserve Board within 60 days.

He said the task force ''has not been asked to review separately takeovers, matters of coporate governance, insider trading or other topics in the securities industry, nor asked specifically to propose new legislation.''

Fitzwater said that ''beyond that, virtually everything is free for them to look at'' - such as the political climate and the budget dispute between the White House and Congress.

The spokesman the group will consider whether, if regulatory changes are recommended, they can be handled by existing self-regulating organizations within the markets, by the Securities and Exchange Commission or by new authorities.

He said the president will evaluate the task force report and ''if appropriate, develop regulatory or legislative proposals to maintain a free, fair and active market.''

He said the group would have the cooperation of the SEC and other executive branch agencies and would report independently to the Federal Reserve.

Reagan announced formation of the panel at his news conference Oct. 22. Asked why it took so long to name the members, Fitzwater said, ''(We) wanted to get the right people, and it didn't take long. It took just the right amount of time.''