The Capitol Visitor Center welcomed its 4 millionth visitor Wednesday, making the summer of 2010 the busiest time period yet for the $621 million complex, according to the Architect of the Capitol’s office.

The milestone comes less than four months after the center logged its 3 millionth visitor in May.

The recent rate of visitors outpaces past high points: It took six months from the CVC’s opening in December 2008 to reach 1 million visitors, then another five months to hit two million in September 2009.

The pace slowed soon after, and the center didn’t log another one million visitors until nine months later, this past May.

The largest ever addition to the Capitol has resulted in doubled visitors. But that success has sometimes rankled Members and tour guides.

In the past, Members complained that new tour schedules and training requirements were curtailing staff-led tours, a long-standing constituent service.

Tour guides, meanwhile, have complained that they have to handle five or six tours per day, sometimes with up to 90 visitors each.

The center has faced security concerns, as well, most recently when a CVC official threw away a bag labeled “anthrax” without notifying police.

The issues came to a head last month when Terrie Rouse, CEO for visitor services at the CVC, was fired after a three-year tenure marred by standoffish relations with Members and allegations of mismanagement.

“When we opened the doors to the Capitol Visitor Center, I noted that visitors now have a respectful and dignified way to come to their Capitol Building, and I invited everyone to come and explore all that the Visitor Center and the U.S. Capitol have to offer,” said Architect of the Capitol Stephen Ayers in a statement. “I’m thrilled that, as of this week, four million people took me up on my offer. We couldn’t be prouder to provide these millions of guests with the wonderful amenities and educational experiences in the Visitor Center on behalf of the Congress.”