It raised $300 million for its debut fund in early 2009, and added third general partner John O’Farrell more than a year later (the founding duo are namesakes Marc Andreessen and Ben Horowitz). It then raised $650 million for its second fund last November, and today is announcing the addition of Scott Weiss as its fourth GP.

Weiss was employee #13 at Hotmail, and stuck around as head of a business development team once the email provider was acquired by Microsoft (MSFT) in 1998. In 2000, he founded and ran networking security company IronPort Systems, which was acquired in 2007 by Cisco Systems (CSCO) for $830 million. He most recently served as vice president and general manager of Cisco’s Security Technology Group.

“I honestly never expected that I’d be part of a VC firm,” Weiss told Fortune earlier today. “I’ve raised dollars from six to eight different VC firms over the years, and the business never really got my blood going until I met Mark and Ben.”

Specifically, Weiss says that Andreessen Horowitz seems to be run more with the innovative rigor of an operating company than one of its investors. For example, he recently mentioned that there should be a mechanism whereby entrepreneurs can review VCs once the deal is exited. His new partners’ response was to begin implementing such a system.

Ben Horowitz says that Weiss brings a different personality and domain expertise to the firm, particularly in terms of sales and marketing distribution strategies. He adds that the firm “has capacity” to add a fifth partner, but that it isn’t in any rush to do so.

For more, read Horowitz’s blog post on the hire.