
 A mysterious collector went on a Dutch buying spree yesterday at Sotheby's, paying $4.27 million for a much-watched Rembrandt portrait and another $1.87 million for a Jan Steen and a Gerrit Dou.   The bidder, identified only as a New York collector and Paddle No. LO179, snapped up the works in the first hour of the old master paintings sale, starting with Rembrandt's "Study of an Elderly Woman in a White Cap," from around 1640. The buyer edged out four other bidders for the portrait, which was reauthenticated as a Rembrandt only recently after a restoration in Amsterdam.   The Rembrandt, depicting a woman wearing a bonnet.  January 27 2006