President Reagan yesterday nominated four men to posts in the Justice Department and named an ultraconservative scholar as assistant secretary of state for human rights and humanitarian affairs.

The State Department post went to Ernest Lefever, whose criticism of the Carter administration's human rights policy was frequent and impassioned, Lefever's appointment was expected (he has been occupying the office most of this month), and has been seen as a gesture to far-right Republicans, who were upset that State had too few of what they regarded as sufficiently conservative officials, as well as a symbol that the new administration would have no truck with the old Carter policy.

Lefever is a private consultant and founder of the Ethics and Public Policy Center.

In the Justice Department, Reagan named Californian Lowell Jensen, district attorney in Alameda County, Calif., as assistant attorney general in charge of the criminal division. Jensen is a friend of White House counselor Edwin Meese III, and the two worked together in the district attorney's office in Alameda County until Meese became legal affairs director for then-governor Reagan.

The president also nominated Rudolph Giuliani as associate attorney general, William Baxter as assistant attorney general in charge of the antitrust division and Theodore Olson as assistant attorney general for legal counsel.

Giuliani, a New York lawyer, is no stranger to the department, having served as an associate deputy attorney general in the Nixon administration. Baxter is a Stanford University law professor who takes a generally narrow view of antitrust law and has testified against legislation intended, on antitrust grounds, to make acquisitions more difficult for oil companies. Olson is a partner in the same Los Angeles law firm that contributed Attorney General William French Smith to the Reagan administration.

Reagan also named Michael Cardenas, a partner with the accounting firm of Fox & Co. in Fresno, Calif., as administrator of the Small Business Administration.