Boston voters will go to the polls today under sunny skies to cast their ballots in the preliminary mayoral election.

The field of four major candidates, who spent yesterday combing the city in search of last-minute votes, will be narrowed to a pair, with a runoff election scheduled for November 6.

Municipal officials yesterday predicted a light turnout, regardless of weather conditions. In the 1975 preliminary election, about 40 per cent of Boston's 250,000 eligible voters went to the polls.

Incumbent mayor Kevin H. White, who spent almost $1 million on the preliminary campaign, spent yesterday wooing elderly voters and pounding the pavement in South Boston and the North End. White, who is expected to lead the field in today's election, turns 50 today.

White's three major challengers toured the city yesterday, hitting key precincts during the day before gathering at a candidates night in the North End.

Three-time challenger State Sen. Joseph F. Timilty will vote at 8:30 a.m. before sweeping through the city's polling places later in the day.

Timilty, who is expected to run second to White, made stops yesterday in Kenmore Square, Brighton and the North End, taking time out from his campaign schedule to accompany his son to school for the first day of kindergarten.

State Rep. Melvin H. King made stops at downtown housing projects for the elderly and in Chinatown yesterday before spending the better part of the afternoon in his State House office.

King said yesterday White runs "a visionless and unethical administration."

"After November 6," King said, "Boston City Hall will no longer be a haven for political hacks and a source of conscript labor to advance the electoral fortunes of the incumbent mayor."

Boston School Committee President David Finnegan, expected to run third in today's election, shook voters' hands at Boston Municipal Court and City Hospital before holding a press conference in front of City Hall.

Field workers in Finnegan's campaign office said yesterday their organization will target its election day efforts on West Roxbury and Dorchester.

Luis Castro, the Socialist Party candidate, and U.S. Labor Party candidate Lawrence E. Sherman are also on today's preliminary ballot.

Boston Election Department figures indicate that about 234,000 citizens are registered to vote in today's election, a lower number than in the last two preliminary elections.

John F. Donovan, Boston's chief registrar, said recently about 43 per cent of the electorate--about 101,000 people--will actually go to the polls.