Stock trading app Robinhood has beaten its rival E-trade when it comes to users, doubling the number of accounts in the past year.

The start-up now has 4 million brokerage accounts on its platform. On Thursday, it announced a $363 million funding round, valuing the company at $5.6 billion — quadruple from about a year ago.

New investors included Sequoia, Google's venture capital fund Capital G and Iconiq. DST Global led this Series D round and had facilitated the $110 million round last year, when Robinhood had 2 million users.

One of Robinhood's biggest competitors, E-Trade Financial, which also has a mobile app, had 3.7 million brokerage accounts at the end of March, with an annualized new account growth rate of 6.6 percent, according to the company's most recent 10-Q filing.

E-Trade Financial did not respond to CNBC's request for comment.