  We re-examine the evolution of neutrino asymmetries in several four neutrino
models. The first case involves the direct creation of $L_{\nu_e}$ by $\nu_e
\leftrightarrow \nu_s$ oscillations. In the second case, we consider the mass
hierarchy $m_{\nu_\tau} \gg m_{\nu_\mu}, m_{\nu_e}, m_{\nu_s}$ where $\nu_\tau
\leftrightarrow \nu_s$ oscillations generate a large $L_{\nu_\tau}$ and some of
this asymmetry is converted into $L_{\nu_e}$ by $\nu_{\tau} \leftrightarrow
\nu_{e}$ oscillations. We estimate the implications for BBN for a range of
cosmologically interesting $\delta m^2$ values. The present paper improves on
previous published work by taking into account the finite repopulation rate and
the time dependence of the distortions to the neutrino momentum distributions.
The treatment of chemical decoupling is also improved.
