Pentagon sources say it is now almost certain that Vice Adm. James A. (Ace) Lyons, a close friend of Navy Secretary John F. Lehman Jr., will get his fourth star this summer and take command of the Pacific Fleet. That is the Navy's only available four-star command that isn't a "unified" post having Army and Air Force contingents. As a result, Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff, would have less say about the appointment. And since Vessey is not thought to share Lehman's admiration for the blunt and beefy admiral, Pac Fleet would be the easiest slot for Lyons to ease into -- jumping ahead of several three-star admirals who have more seniority.

None of that becomes official, however, until the Pentagon finds out how long Vessey will stay.

If Vessey goes, the Navy assumes that an admiral should become chairman of the joint chiefs, after a run of Army (Vessey) and Air Force generals (David C. Jones and George S. Brown) who have held the post. Even if the Navy wins for the first time since Adm. Thomas Moorer retired in July 1974, however, there's no agreement on who the lucky admiral would be. Adm. James D. Watkins, now chief of naval operations, reportedly wouldn't turn the job down. But Adm. William J. Crowe Jr., commander-in-chief of all Pacific forces, also is considered a candidate. Crowe is said to have won the respect of Defense Secretary Caspar W. Weinberger for his intelligence and knowledge of his command, but he was passed over for chief of naval operations -- and the unified commands in the field generally do not lead to higher things. HIGH ROAD TO CHINA? . . .

Vessey gave a hint of what it means to be the nation's top soldier during his testimony to the House Armed Services Committee yesterday. Discussing the quality of the reserve forces, the general said he had reason to be thankful for their high degree of readiness last month when his plane experienced mechanical difficulties in Alaska en route to China.

Vessey said there was an Air Force Reserve C141 transport plane at the same base. As he explained it: "I said, 'Where are you guys going?' " The crew said they were headed home to California. "I said, 'No you're not. You're going to China.' " MEANWHILE, ON THE HILL . . .

Two new Republican members will tug the Senate Armed Services Committee sharply to the right. Sen. Jeremiah Denton of Alabama, a retired admiral, Vietnam prisoner and former committee member, is back on board, and new Sen. Phil Gramm of Texas won a seat. Democrats, meanwhile, are hoping to add one member to the committee, but are divided between Sen. John Glenn of Ohio and freshman Sen. Albert Gore Jr. of Tennessee.

There's some turmoil on the House side, too, where the Military Reform Caucus is wrangling over who should be its chairman. Rep. James A. Courter (D-N.J.) has been in charge for the past two years, and the caucus' short tradition suggests it's time for him to step aside. That's what Sen. Gary Hart (D-Colo.) and Rep. G. William Whitehurst (R-Va.) did after they co-chaired the caucus during its first two years, allowing Sen. Nancy Landon Kassebaum (R-Kan.) and Courter to take over in 1983. Now Kassebaum has stepped down in favor of Sen. David H. Pryor (D-Ark.), and Hart is urging Courter to do the same.

Courter said yesterday "there is no established tradition. It is the members of Congress who will make the decision."