  We describe the research and development program carried out by the UCLA -
Torino group leading to the ZEPLIN II detector under construction for the
Boulby Laboratory. Knowledge of ZEPLIN II performance will help in the design
and construction of ZEPLIN IV. This detector could be located at a U.S.
underground laboratory (WIPP site or others) or elsewhere. We show that a
detector of this size is required to observe SUSY WIMPS.
