Similar increases will apply to magazines and advertising mail, the Postal Regulatory Commission said.
The hike, raising a first-class stamp to 49 cents from 46 cents, starts Jan. 26 but is temporary, lasting "just long enough to recover the loss," Chairwoman Ruth Goldway said in announcing the increase.
She predicted "just long enough" would be close to two years.
The independent regulatory commission determined the Postal Service's loss, caused by a substantial 25.3 billion-piece drop in mail volume, was $2.8 billion between 2008 and 2011, Goldway said.
The commission in its 2-1 ruling rejected the Postal Service's request to make the increase permanent, saying the service "failed to provide justification for permanent price increases in connection with recession-related mail volume losses."
It added the service "conflated losses that are a result of Internet diversion with losses that were a result of the Great Recession" and said the additional postage revenue should cover only recession losses, not deficits stemming from email and commercial package-delivery competition.
The post office said last month it lost $5 billion in the fiscal year that ended Sept. 30. The year before it said it lost a record $15.9 billion.
The 4.3 percent emergency, or "exigent," increase the commission approved Christmas Eve comes on top of a 1.7 percent inflation increase approved last month. That brings the total increase for most mail to 6 percent.
The commission must approve postage increases above those tied to inflation.
"The Postal Service is disappointed in the Postal Regulatory Commission's split decision to limit the duration of a modest exigent rate increase," the service said in a statement, adding it was reviewing the ruling "in an attempt to determine the basis for the decision."
The trade group MPA, the Association of Magazine Media, called the increase a "counterproductive decision" that "does nothing to fix [the Postal Service's] systemic problems," while "hurting consumers, forcing layoffs and impacting businesses."
The last time a first-class stamp rose 3 cents was June 30, 2002, when it went to 37 cents from 34 cents.
A first-class stamp has been 46 cents since Jan. 27, 2013.