I believe they fought in singles at Starrcade 1996, Superbrawl VII, and Halloween Havoc 97 (in a steel cage). I believe Starrcade and HH were non-title (both which Piper won), but I think Superbrawl was for the title (which Piper lost courtesy of Savage joining the nWo that night). I forgot the angles for the PPV matches, someone please help. They also had tag matches against each other at Spring Stampede 98 and GAB 98 in which both Hogan and his partners (Nash then Hart) won.
They had four if I remember correctly:
1 - 7/20/1998 in Salt Lake City, UT on Nitro for the vacant US title, Bret wins and becomes the WCW United States Champion for the 1st time in his career.
2 - 10/26/1998 in Phoenix, AZ on Nitro, DDP wins via Diamond Cutter - TITLE CHANGE -
3 - 11/22/1998 in Auburn Hills, MI at World War 3 (which was also the PPV's main event), DDP wins via The Diamond Cutter - TITLE RETAIN -
4 - 11/30/1998 Chattanooga, TN on Nitro, Bret wins via The Sharpshooter on an unconscious DDP after the Giant interfered in the match which was No-DQ. - TITLE CHANGE - This led to the Page / Giant match at Starrcade while Hart was recovering from a pulled groin.