The long-overdue fourth National Energy Policy Plan is making yet another trip through the typewriter. The biennial report, which is mandated by the act that established the Energy Department, originally was due April 1.

But Congress agreed to postpone that deadline until July 29 so the plan could better reflect the views of Secretary Donald P. Hodel, who was relatively new in the job when the report was first due.

When Hodel reviewed the final draft of his views, however, he reportedly did not like much of what he read.

Energy Department sources now say the plan probably will be ready in mid-September.***

FOREIGN AFFAIRS . . . Helmut Merklein, a professor of petroleum engineering at Texas A&M University, has surfaced as the latest candidate for the position of assistant secretary of energy for international affairs.

The slot has been vacant since Henry Thomas resigned in November, and with each passing day morale in his old office hits a new low. Despite Hodel's pledges to strengthen the division, the staff, which numbered 150 when President Reagan took office, has dropped to about 50.

The Energy Department, however, is acquiring some supplemental international affairs expertise.

The new White House fellow assigned to DOE for the 1983-84 year is Maj. George Selden Jr.

Selden, 36, is an Army specialist in Soviet and Warsaw Pact military affairs.***

WASTE NOT, WANT NOT . . .Another vacancy that seems to produce a rumor a week, but no firm candidates, is the position of director of the nuclear waste program.

This office, which has never had a permanent director, was set up to implement legislation passed last December calling for creation of a permanent burial site for spent fuel from the nation's atomic power plants.

Now, backers of Edward Davis, senior vice president of the American Nuclear Energy Council, an industry group that lobbied hard for the bill, are making a new effort to get him named to the post.

In response, the Nuclear Information and Resource Service, an organization often critical of the nuclear industry, yesterday issued a press release terming Davis "a totally inappropriate choice." ***

LOCAL REACTION . . . Despite Hodel's lack of enthusiasm for the Clinch River breeder reactor, the lobby of the Forrestal Building these days sports an elaborate exhibit touting the project with a scale model of the advanced nuclear power plant as its centerpiece.

The exhibit, which had been languishing in a warehouse, was moved into its position of prominence after President Reagan personally blessed a major effort to obtain the funds necessary to complete the project when Congress returns in September.***

DELAYED REACTION . . . One of the losers disappointed by Hodel's decision to locate a new nuclear reactor that will produce plutonium and tritium for the nation's weapons program in Idaho has not come up empty-handed after all.

Richland, Wash., home of the so-called Hanford reservation, the sole source of plutonium for the weapons program in the 1940s and early 1950s, is going to get a $600 million facility that will use the atomic vapor laser isotope separation process to purify plutonium.

This process, being developed by the Lawrence Livermore National Laboratory, will be used at Hanford to purify the DOE's stockpile of 15 tons of fuel-grade plutonium and turn it into a higher-quality material suitable for weapons.