How would the existence of a 4th neutrino affect cosmology?

The ratio of the primordial helium to hydrogen, and deuterium to hydrogen are already consistent with exactly 3 flavors of neutrinos.

If one more is found, then that means that we do not understand Big Bang nucleosynthesis and that Big Bang cosmology MAY be in error in a very fundamental way. It also means that physicists who now think they understand low-energy physics and the quark-lepton symmetry are in for a big surprise. So far, there is no evidence for a 4th type of neutrino...but people are looking because the pay back for moving our understanding of cosmology and physics is so great.