Flavor changing neutral currents with a fourth family of quarks

For a model with a fourth family of quarks, new sources of flavor changing neutral currents are identified by confronting the unitary 4x4 quark mixing matrix with the experimental measured values of the familiar 3x3 quark mixing matrix. By imposing as experimental constraints the known bounds for the flavor changing neutral currents, the largest mixing of the known quarks with the fourth family ones is established. The predictions are: a value for |V_{tb}| significantly different from unity, large rates for rare top decays as t->c\gamma and t->cZ, the last one reachable at the Large Hadrion Collider, and large rates for rare strange decays s->d\gamma and s->dg, where g stands for the gluon field.