The biggest non-event of the last four weeks is the failure of Ronald Reagan to change his Cabinet. That inaction may tell us more about his future plans than all of the announcements and trips he is making.

If he is planning to run for reelection, as he wants us to think, then this is the time to strengthen his Cabinet. The mid-term hiatus gives the departing members a graceful exit cue, and it allows the newcomers at least a year's time to make their marks before the formal onset of the presidential campaign.

At the top levels of the White House, no one disputes the need for new blood and new energies. They just say "it can't happen." Except for the shift of Donald Hodel from the No. 2 job at the Interior Department to succeed retiring Secretary of Energy James Edwards, no changes are planned or expected.

Yet the political weakness of the Cabinet is a constant source of complaint within the White House. "We've got a secretary of labor who can't get anyone on the phone at the AFL-CIO," said one lamenting Reaganite last week. "We've got a secretary of housing and urban development who has no allies in city hall or the home-building industry. We've got a secretary of interior whose actions have raised more money for anti-administration environmental groups and candidates than those people have ever seen before."

And, he might have added, Reagan has about nine other guys who are all middle-aged, white, male Anglo-Saxon Protestants, at a time when the administration is trying desperately to shed its image of stereotyped narrowness.

Cabinets, by tradition, are both governmental and political institutions. The top officials at State and Defense are usually exempted from political chores, even though their policies can be of the greatest political significance. But when an administration reaches its mid- point, it is not only permissible but mandatory for the rest of the Cabinet to pull its political weight.

In the mid-term campaign, the Reagan Cabinet was found politically wanting. "Where do you send these people where they'll do you more good than harm?" asked one White House political operative.

Why will Reagan not make any changes? The answer, one is told, is that he will not fire anyone who has not been flagrantly disloyal to him or his policies. He won't even promote the duds into comfortable ambassadorships. This is said in a resigned tone, by people who half admire, half regret the mixture of stubbornness and principle that stays Reagan's hand.

But an outsider must wonder if there is not something more at work here. When Reagan last month asked his close friend, Sen. Paul Laxalt, to take on a newly created, part-time position as "general chairman" of the Republian National Committee, the move was designed -- and generally accepted--as a signal that Reagan is preparing to run in 1984.

It could mean that. And it could also mean that Reagan wants us to think he's going to run again. It serves either purpose equally well.

Restaffing the Cabinet for the 1984 battles -- trimming the deadwood and bringing in some people with political smarts -- would be a stronger indication that Reagan really intends to run.

Maybe Reagan is unaware of the political realities of which the people around him in the White House speak so freely. But I find that hard to accept. Or maybe he doesn't care about the political strength of his Cabinet, because he doesn't plan to run again.