  We consider a quadrilateral 'mini' finite element for approximating the
solution of Stokes equations using a quadrilateral mesh. We use the standard
bilinear finite element space enriched with element-wise defined bubble
functions for the velocity and the standard bilinear finite element space for
the pressure space. With a simple modification of the standard bubble function
we show that a single bubble function is sufficient to ensure the inf-sup
condition. We have thus improved an earlier result on the quadrilateral 'mini'
element, where more than one bubble function are used to get the stability.
