Tsai has bought about $151 million worth of Alibaba’s U.S.-traded shares in the fourth quarter, via his Blue Pool Management family investment vehicle, a securities filing confirmed on Tuesday. Ma, who stepped down as the company’s executive chairman in 2019 but remains a major shareholder, bought $50 million worth of Hong Kong-traded stock in the quarter, according to a person with knowledge of the matter. (Both men already hold sizable amounts of Alibaba stock.) The purchase sizes aren’t huge — Alibaba’s market capitalization is about $171 billion — but given…