WASHINGTON – Hillary Rodham Clinton formally resigned Friday as America’s secretary of state, capping a four-year tenure that saw her shatter records for the number of countries visited. John Kerry was sworn in to replace her.

In a letter sent to President Barack Obama shortly before she left the State Department for the last time in her official capacity, Clinton thanked her former opponent for the 2008 Democratic presidential nomination for the opportunity to serve in his administration. Clinton said it had been an honor to be part of his Cabinet. “I am more convinced than ever in the strength and staying power of America’s global leadership and our capacity to be a force for good in the world,” she said.

Her resignation became effective at 4 p.m. EST, when Supreme Court Justice Elena Kagan swore in John Kerry as the top U.S. diplomat. The former Massachusetts senator and 2004 presidential candidate is the 68th secretary of state.