The social media company announced late Monday it is buying MoPub, a mobile-focused advertising exchange, in its fourth major purchase of the year.

Terms of the deal were not disclosed. Published reports put the price at about $300 million or more, based an estimated values of Twitter’sprivately-held stock that is being used to make the purchase.
Twitter’s statement said that MoPub’s technology lets mobile application publishers manage their inventory of multiple sources of advertising.

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Twitter, which is believed to be weighing an initial public offering some time in 2014, has been buying a number of tech startups as it tries to transform itself into more of a multimedia hub.