As President Reagan and Soviet leader Gorbachev sit down in Moscow at their fourth and final summit meeting, public expectations have swung from hope to disillusionment.

At last December's glowing summit in Washington, the two superpowers pledged to build upon the new treaty eliminating short- and medium-range missiles with another one cutting in half the number of long-range nuclear weapons.But it wasn't until a few days ago that Washington and Moscow would admit publicly they still aren't even close to such an ambitious agreement, let alone having the pact ready for the current summit.

If the so-called START (for strategic arms reduction treaty) is not signed and ratified before President Reagan leaves office next January, there's bound to be an extra delay. The new American president - Republican or Democrat - would want to scrutinize it closely.

But so be it. It's far better to take whatever extra time may be needed than to rush into a flawed START pact just to maintain the momentum of East-West negotiations.

The main sticking points in these negotiations seem to involve sea-based cruise missiles, mobile missile systems, and the Strategic Defense Initiative (SDI) or Star Wars. The Soviets insist that sea-based cruise missiles can be effectively verified; Washington maintains they cannot. The Soviets expect fewer problems with the issue of mobile missiles. Though Moscow no longer demands the abandonment of Star Wars, it insists that work on SDI not exceed the bounds of the 1972 interpretation of the Anti-ballistic Missile Treaty.

The Soviets, however, have repeatedly given ground in response to firm stands by President Reagan - a point that should be kept clearly in mind by whoever succeeds him.

Meanwhile, all the attention given to nuclear arms control should not detract from the opportunity to work at the summit on reducing the vast arsenal of conventional, or non-nuclear, weapons.

In some ways, conventional arms pose more problems for negotiators than nuclear arms. For one thing, there are far more of them. For another, their wide variety makes it harder to compare the relative military preparedness of the Free World and the communist bloc. While some analysts see the Soviets as having the most troops and weapons, others insist the West has the advantage because of such intangibles as troop morale, the quality of its military hardware, plus superior communications and civilian support.

Moreover, though dramatic breakthroughs are not expected at the Moscow summit, it provides an unexcelled means for getting the world's attention. For President Reagan's part, he should use this forum to remind Russia that the Berlin Wall is still repugnant to free people everywhere and to keep demanding an end to Soviet international expansionism.