Few could fail to notice that the four senators receiving comparatively mild treatment from the Senate Ethics Committee Feb. 27 were the four who are expected to seek re-election.

The committee singled out the fifth, Democrat Alan Cranston of California, for heavy criticism and further proceedings. Cranston, undergoing treatment for prostate cancer, is the only one of the five who has announced he will retire.But the committee's "no further action required" verdict may not have been quite the political "all clear" desired by the other four senators: Republican John McCain of Arizona and Democrats Dennis DeConcini of Arizona, Donald W. Riegle Jr. of Michigan and John Glenn of Ohio.

Political opponents and observers in their home states warn that none should consider himself immune to a harsher verdict from the voters.

On the home front, some problems already are apparent: Low polls, leaner campaign bankrolls and the nagging association of their names with the continuing troubles of Charles H. Keating Jr., the savings and loan industry and the financial system.

The four may even run the risk of becoming symbolic targets of a broader disenchantment with Congress.

Ironically, the first senator to test the scandal's potency at the polls could be McCain, whose involvement in the regulatory affair was arguably tenuous. There had been speculation in Arizona that McCain would be exonerated completely, a possibility McCain himself had held out.

"It's a setback for him, there's no question about it, and it will affect his candidacy next year," said Charles W. Pine, an Arizona public relations consultant and newsletter publisher.

Anita Dunn, director of communications for the Democratic Senate Campaign Committee, notes that favorability ratings have doubled during the gulf war for McCain, a prisoner of war in the Vietnam War. But she adds that even after doubling, those ratings remained well below 50 percent.

With both McCain and DeConcini involved, the issue has had more intensive media coverage in Arizona than in any other state.

The only other member of the Keating Five scheduled to face the voters in 1992 is Ohio's Glenn, who has won each of his terms to date with at least 62 percent of the vote.

The former astronaut and presidential hopeful will be 71 in 1992, and it could be that the added aggravation of the Keating affair will have dulled his appetite for more politics.

Riegle has a history of escaping voter wrath. Yet this might be a scandal that even Reigle can't walk away from unblemished.