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   We propose ASCA observations of four clusters of galaxies found to be
   among the most X-ray luminous clusters in the sky in the ROSAT All-Sky
   Survey. These systems form part of a statistical sample of massive
   clusters selected from the XBACs and BCS cluster samples for an in-
   depth study of the mass distribution in clusters by combining X-ray
   mass determinations with those from observations of strong
   gravitational lensing. All targets proposed here for observation show
   giant gravitational arcs in deep CCD images obtained from Mauna Kea.
   Our combined X-ray/lensing approach will provide the most accurate and
   reliable measurement of cluster masses for a statistical sample to date
   and will allow a first determination of the cluster mass function.
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