SAN FRANCISCO (AP) _ President Reagan's approval rating has fallen below 50 percent in his home state, its lowest level in nearly four years, according to a poll.

''The Teflon might be wearing off,'' said pollster Mervin Field, referring to Reagan's reputation for not having political and policy problems stick to him in the public's perception.

The California Poll released Tuesday showed Reagan receiving a good-to- excell ent rating from 45 percent of the 1,026 adults surveyed from April 3 to 10 and a poor rating from 26 percent.

They were the lowest ratings since October 1983, when Reagan received a 44 percent excellent-to-good rating and a 26 percent poor rating.

Last May, Reagan received an excellent or good rating from 60 percent and a poor rating from only 15 percent, a positive margin of 4-to-1 in the California Poll.

The lowest rating of his presidency came in March 1982, during a national recession, when he received a 37 percent excellent-good rating and a 32 percent poor rating.

Although a majority in the latest survey expressed confidence that Reagan will make the right decisions on major issues, 41 percent said they were not confident of the president's leadership in domestic problems and 45 percent were not confident about his leadership in international problems.

The poll reported a margin of sampling error of plus or minus 3.3 percentage points.

In another poll, an ABC News-Washington Post telephone survey of 1,509 people published today indicated that Reagan's attempts to put the Iran-Contra affair behind him haver not been successful.

The poll showed about the same job rating for the president as one taken March 19, just after his news conference on the Tower report - 50 percent approved of his job then and 48 percent in the latest poll, a drop pollsters found statistically insignificant. The same pollsters showed Reagan at his low point in a Feb. 26 survey that gave him 44 percent approval. His rating before the Iran-Contra affair was 67.