They are leaving nearly 50 years of combined House experience, making for the region's largest cumulative loss of tenure since the early 1990s.
The changes seem to reflect a growing malaise among House members sick of a dysfunctional Congress. Even lawmakers who had attained coveted positions have seen their pull diminished, as most legislation is handled by crisis or not at all - a trend that may, ironically, blunt some of the impact of losing veteran lawmakers.
A seat on a key congressional panel does not ensure the influence it once did.
"If you're on one of these committees," said U.S. Rep. Chaka Fattah (D., Pa.), a 10-term incumbent, "it can be quite frustrating."
Three local House seats changed hands in the tea-party wave of 2010, but the last time four of the region's seats turned over between the start of one Congress and the next was in 1993 (though the comparison isn't entirely parallel, because the districts' boundaries were different then).
Before that, you'd have to search back to the post-Watergate elections in the mid-1970s or the social upheaval of the late 1960s to find such sweeping turnover in the region in one election cycle.
For new House members joining the fray next year, there will be a steep learning curve and potentially years of work to land a seat on an A-list committee.
"It's drinking out of a fire hose. I can remember that in the first orientation week," Gerlach said. "Those that have had legislative experience in the past, particularly at the state level, better understand it" at first.
"It can take two years, it can take 10 years," Fattah said. "It has to do with getting on the committee or jurisdiction where you have some expertise or you can develop some expertise."
The Philadelphia Democrat started out on a panel that handled postal issues before moving to the education committee and then the coveted Appropriations committee, which doles out federal dollars.
U.S. Rep. Chris Smith (R., N.J.), who joined Congress in 1981, is the longest-serving lawmaker from the region. He said it would take at least one term to get up to speed, and even more time to see major initiatives become law. He's still working to pass one bill he first developed seven years ago.
Along the way, he said, he has had the advantage of learning from chairing more than 500 hearings. "You get a depth of knowledge on the issues that can't be gleaned out of a book," Smith said.
Now one of the top members of the Foreign Affairs committee, Smith had just finished a news conference with a Vietnam expert he has known for 20 years.
"You learn who has good information, who you should listen to and not," Smith said. "That doesn't happen in your first term."
Among those leaving, Gerlach and Schwartz both have seats on the powerful Ways and Means Committee, which writes tax policy. Andrews was a close ally of Minority Leader Nancy Pelosi.
Schwartz and Andrews had key roles working on President Obama's signature health law - a measure affecting one of the Philadelphia region's most vital business sectors, Fattah said.
Former U.S. Rep. Patrick Murphy, a Bucks County Democrat, recalled how the late Jack Murtha, the longtime dean of the Pennsylvania delegation, would pull together lawmakers from the state to get officials from both parties using their pull on key regional issues.
"That seniority that we're losing is really going to hurt our clout," Murphy said.
But Gerlach and Runyan both cited frustrations with Washington's knotted politics when they announced that they were leaving the House. Schwartz is seeking a job with far more influence than she would have in the minority of the a GOP-controlled House.
The influence conveyed by seniority has diminished as work in Congress has bogged down.
The few significant bills that have cleared both houses in recent years have usually been "must-pass" measures thrown together to defuse impending crises. They have usually been crafted in closed-door meetings of the top leaders in each chamber, rather than going through the committee process where rank-and-file lawmakers can leave an imprint. Most other major legislation has been stalled between the GOP-controlled House and the Democratic-led Senate.
Ethics reforms to eliminate "earmarks" - federal money designated for cherished projects - has also dampened the power of individual lawmakers to bring funding back to his or her district.
"Committees and seniority mean less today than they once did," Donald Wolfensberger, a congressional scholar now with the Woodrow Wilson International Center for Scholars, wrote in an e-mail.
For years, the Philadelphia-area delegation has been remarkably stable. Starting in November 2000, the 10 seats in the region have each had seven elections. Of those 70 contests, only eight resulted in a seat changing hands.
Smith's district has set the standard for constancy. The seat, which long included part of Burlington County but no longer does, has had only two occupants since 1955 - him and Frank Thompson.
The neighboring South Jersey district now represented by Runyan once had that kind of permanence: U.S. Rep. Jim Saxton served there for nearly a quarter of a century.
But with Runyan's departure, the district next year will have its fourth member of Congress in the last five terms.