  The conformal geometry of spacelike surfaces in 4-dimensional Lorentzian
space forms has been studied by the authors in a previous paper, where the
so-called polar transform was introduced. Here it is shown that this transform
preserves spacelike conformal isothermic surfaces. We relate this new transform
with the known transforms (Darboux transform and spectral transform) of
isothermic surfaces by establishing the permutability theorems.
