WITH AN ENDOWMENT that has bounded to more than $4 billion, Harvard could do away with tuition anytime it wants. There's no sign it will want to any time soon.

But thanks to a decision to tap the endowment's revenue a bit more energetically next year than in years past, undergraduates will pay just a little less than previously thought for coming semesters on the Charles. (Don't worry, tuition will increase, just not as much as everyone thought.)

In years past, Harvard has plowed most of its profits back into the endowment. The University will retreat from that conservative stance just a shade next year, spending $55.3 million rather than the planned $54.3 million on the yearly costs of keeping the College running.

That's an almost indiscernible step in the right direction. It doesn't go anywhere near far enough toward correcting the historic failure of Harvard's endowment to cover a fair share of the cost necessary to keep this place running.

Underneath reams of financial records churned out by University number-crunchers, a few simple facts are hidden. In 1947 undergraduate tuition took care of about 20 percent of Harvard's expenses. today, tuition covers nearly 30 percent.

But while undergraduates have taken up an increasing portion of University expenses--and signed for more and more loans--the share of expenses which the endowment pays has plummeted. This has happened during a 10-year period that has seen the endowment more than sextuple.

Why hasn't a much-multiplied endowment taken over some of the expenses students are asked to bear? At a time when the loan burdens of students are restricting their future options more than ever before, financial office explanations don't really satisfy.

The federal government has increased student aid only minimally over the past decade, and now the Reagan Administration is intent on slashing it. In addition, the composition of that aid has been changing to the detriment of the two-thirds of Harvard undergraduates who receive financial aid. For the last 10 years, grants at Harvard have grown at a slower rate than educational costs, so loans and employment have had to make up the difference.

No reasonable person would suggest that the long-term health of the endowment be sacrificed for the sake of a generation of students. However, reasonable people in the financial office evidently have forgotten that it is no more desirable to sacrifice the financial health of undergraduates for the sake of the endowment's never-ending multiplication. They should correct this--and soon.