Four members of the Securities and Exchange Commission have broken with their chairman and recommended a "substantial increase" in the agency's personnel to "keep pace with the rapidly changing industry we regulate."

Last Friday the four commissioners wrote Rep. Timothy E. Wirth (D-Colo.), chairman of the subcommittee that oversees the SEC, saying "our consensus is that a 4 percent increase in staff positions . . . is necessary." SEC Chairman John S.R. Shad has called for a 6 percent decrease.

There are 2,021 staff positions at the SEC. The 1984 budget proposes to reduce that to 1,896, while the four commissioners believe the level should be 2,099.

Wirth made the correspondence public yesterday. The letter is the latest communication in a budget brouhaha involving the agency.

It began in February when Shad told the House Appropriations Committee that, as the result of increased efficiency, the SEC could get along with just a 3 percent increase in its 1984 budget to $92 million. That would mean a reduction of 125 jobs, or a 6 percent reduction in personnel.

Soon thereafter, Wirth sent Shad a 15-page letter in which he accused the chairman of appearing to favor budget cuts over protecting the public from investment fraud.

Meanwhile, Wirth asked the four commissioners, who had not participated in the original budget discussions, to examine the budget projections and "make an independent assessment of the SEC's needs."

After meeting with the SEC staff, John R. Evans, Barbara S. Thomas, Bevis Longstreth and James C. Treadway Jr. responded. "We have concluded that a 6 percent cut in staff positions would make it exceedingly difficult to maintain a satisfactory presence in each of the important areas of our responsibility," they wrote. "At the present funding level, we are stretched thin--so thin, in fact, that we have concluded it is the minimum responsible staffing level."