An unhappy milestone was passed this week - a milestone that reflects the tendency of federal efforts to outlast their usefulness.

It was passed Wednesday when Lawrence E. Walsh completed his fourth year as special prosecutor in the old Iran-Contra scandal.Though Walsh's efforts have produced little in relation to the time, effort and money expended on them, he insists on pursuing appeals that seem unlikely to pay off.

So far, Walsh has spent $23 million investigating and prosecuting various officials involved in the sale of U.S. arms to Iran and the diversion of profits to the Contra rebels in Nicaragua. That's more than the cost of any other court-appointed special prosecutor, including the one in the Watergate scandal.

In return, Walsh has gained eight convictions, five by negotiated guilty pleas. But even those victories, such as they are, are starting to turn into ashes.

Former White House aide Oliver North put a big hole in Walsh's record earlier this year when a federal appeals court set aside North's three felony convictions for his role in covering up the Iran-Contra scam.

The decision bodes ill for Walsh's other major triumph - the five guilty verdicts earlier this year against former national security adviser John M. Poindexter, convicted of lying to Congress and obstructing its investigations.

Walsh is like the dinner guest who won't go home after the party is over. It's time to move on and let historians fill in the gaps in the Iran-Contra scandal as pertinent information becomes available. This sad chapter in America's history was not meant to provide a permanent job for the special prosecutor.