  We find a minimal set of constraints which are independent of the choice of
weak quark basis and necessary and sufficient for CP conservation for four
quark families, including also the case of degenerate quark masses. These
invariant conditions are written in the mass eigenstate basis as a function of
the fermion masses and charged current mixings. CP violation is then related to
the areas of three unitarity quadrangles and the CP violating effects of the
fourth family are discussed in the case of small mixings.
