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10 months after mobile payments startup Square raised $100 million at a valuation of $1 billion, the company is now said to be seeking a new round that could value it at up to $4 billion, reports All Things Digital.

ATD’s Tricia Duryee cites multiple sources for the news, but she also notes that the deal is still in negotiations, which means investors could end up valuing the company for less. Still, even at a valuation of only $3 billion, Square’s growth would be impressive. The company, not surprisingly, hasn’t commented on the news.

Former VentureBeat executive editor Owen Thomas pointed out several days ago that Square CEO Jack Dorsey made a trip to Baltimore, and he later confirmed that Baltimore-based Legg Mason was “looking into Square.”

Square started out by offering a credit card reader that plugs into your phone’s headphone jack, letting individuals and small businesses accept card payments on the fly. More recently, the company launched a virtual wallet (“Pay with Square”) and register app for the iPad.

The company hasn’t yet divulged its financials, but Duryee estimates that it could be earning annual revenue of around $122 million. Square takes a 2.75 percent cut from swiped credit card transactions, and 3.5 percent plus 15 cents from keyed in transactions. The company is likely handing over most of its earnings to credit card companies for processing fees, Duryee notes.

Square has big plans ahead: The company hopes to expand internationally, and it has hired former PayPal exec Alyssa Cutright to lead the effort. It has also brought on Google Wallet co-founder Rob von Behren, a sign that it could be interested in near-field communications (NFC) technology.

San Francisco, Calif.-based Square has raised around $137 million so far from investors including Kleiner Perkins, Tiger Global Management, Sequoia Capital, and Khosla Ventures.

Update: DealBook’s Evelyn Rusli reports that Square may be seeking up to $250 million in this round.

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