By Phil Stewart and Andrea Shalal-Esa

WASHINGTON, Oct 10 (Reuters) - Deputy Defense Secretary Ashton Carter, known for his deep knowledge of U.S. defense spending and the defense industry, said on Thursday he was stepping down in December after four years in top Pentagon jobs.

Defense Secretary Chuck Hagel said he "reluctantly accepted" Carter's decision to leave the post.

Carter brought fresh analytical rigor to the job, but also helped reopen lines of communication with the defense industry, said Brett Lambert, who worked closely with Carter before retiring in August as the Pentagon's head of industrial policy.

But Carter's main legacy was his "unwavering, untiring and overwhelming" commitment to making sure that U.S. troops had the equipment to do their jobs, Lambert told Reuters.

Over the years, that meant researching and sending in an array of unusual equipment - from explosive-sniffing dogs to surveillance blimps and mine-resistant trucks that could climb the mountainous roads in Afghanistan.

"I truly believe he saved lives over there," Lambert said.

It was unclear who might replace Carter, although several names surfaced late Thursday as possible successors: Navy Secretary Ray Mabus, former Air Force Secretary Michael Donley and the Pentagon's former policy chief, Michele Flournoy.

Another possible contender might be Linda Hudson, a veteran defense industry executive who has announced plans to retire early next year as chief executive of BAE Systems Inc, the U.S. unit of Britain's BAE Plc.

As deputy defense secretary over the past two years, Carter helped ensure a smooth hand-off from then Defense Secretary Leon Panetta to Hagel.

"He possesses an unparalleled knowledge of every facet of America's defense enterprise, having worked directly and indirectly for eleven secretaries of defense over the course of his storied career," Hagel said in a statement.

Some had speculated that Carter, who has a doctorate in theoretical physics from Oxford University, would be tapped to head the Department of Energy, but Obama administration officials asked him to stay on at the Pentagon to ensure continuity.

Loren Thompson, a Virginia-based defense consultant, said Carter's departure robbed the Pentagon of an experienced manager and could also open the door for mergers among bigger defense companies.

"Carter was the policymaker who said no mergers between top-tier defense contractors. That prohibition is likely to leave with him," he said.

Before becoming deputy defense secretary, Carter was the defense undersecretary for acquisition, technology and logistics, wrangling with complicated weapons programs such as Lockheed Martin Corp's F-35 fighter program.