LE MARS, Iowa (AP) _ Westmar College trustees voted overwhelmingly Saturday to become a branch of Teikyo University of Japan, a merger that Westmar's president called a marriage but four trustees protested by resigning.

More than two-thirds of the 37-member board approved the merger after a six-hour session, college President Arthur Richardson said at a news conference late Saturday. He declined to disclose the actual vote count.

The agreement will bring a substantial dowry to the tiny, century-old college and gives Tokyo-based Teikyo its third branch in the United States. Teikyo has nine other campuses in Japan, and branches in England and Holland.

Robert Rydell of Des Moines, chairman of the board of trustees until he and three other members resigned to protest the merger agreement, said he was ''terribly disappointed'' by the move.

''They have called it a marriage. But in my judgment the engagement period was way too short. We didn't have a chance to get acquainted,'' he said.

Richardson said the Board of Trustees would dissolve itself in about 10 days and re-form with at least seven members, the majority of whom will be Americans.

The acting chairman of the new board will be Takashi Yamanaka, chairman of the executive committee at Teikyo's Loretto Heights University in Denver.

The agreement clears the way for up to 200 Japanese to arrive on the 62- acre campus in April or May and join Westmar's 566 students when classes begin in September, Richardson said. He said up to 200 more would arrive in Le Mars, about 20 miles northeast of Sioux City, for the fall term in 1991.

Teikyo has an enrollment of more than 13,000 students. It already has agreements to buy Loretto Heights from Regis College and to buy Salem College in Salem, W. Va.

''Iowa is 'down to earth' and its people have very healthy minds. I'd like to introduce that to Japanese. It's important to introduce your Iowa values,'' Yamanaka said earlier.

Faculty, students and trustees were disappointed Richardson didn't tell them what was going on until last month. The merger talks began in November.

''It did kind of happen all of a sudden,'' Richardson said. ''But you can't announce a gift you don't have.''

On the edge of financial collapse in the mid-1980s when enrollment bottomed out at 417 students, Westmar has been searching for ''some kind of international arrangement,'' he said.

Others worried that the United Methodist Church would sever its relationship with Westmar.

The college, celebrating its centennial this year, receives about $90,000 annually from the church.

Gregory Clapper, a Methodist minister and the school's director of religion and philosophy programs, said the school's mission is more important than money.

''What has always been more important for many people than the money which the institution gives to Westmar is the simple, powerful, symbolic statement: 'We are a United Methodist College,''' Clapper wrote in a letter published in the Le Mars Daily Sentinel.

Le Mars Chamber of Commerce President John Boyd, a 1967 graduate of Westmar, said the Japanese students would help the local economy.

''For every $1 spent, that would be turned over seven times. If a student from Westmar comes in and spends $100 a month, they're infusing $700 into our economy,'' Boyd said.

Other residents want the Japanese to stay away.

''Our leaders in Washington have been giving in to Japan. Our ex-president even went over and got paid off after he got out of office. Also, why should we pay for all the military protection for Japan and then let Japan come over here and buy us out?'' Don Eickholt of Hinton said in a letter to the Sentinel, referring to a recent trip by former President Reagan paid for by a Japanese company.

''I believe they want students schooled in this country so they will be able to take over,'' his letter said. ''The younger generation think dealing with Japan is a way to make a buck, and that is important to them. Who won World War II, I wonder.''