  In the framework of the mass mixing scheme we have considered mixings and
oscillations of $\nu_e, \nu_\mu, \nu_\tau$ neutrinos and obtained expressions
for angle mixings and lengths of oscillations in dependence on components of
the nondiagonal mass matrix. Then analysis of these obtained results was done
by using modern experimental data on neutrino oscillations. It has been shown
that in this approach the lengths of neutrino oscillations $L_{2 3}$ and $L_{1
3}$ are not compulsory to be equal. It means that the angle mixing $\theta_{1
3}$ can be not very small, i.e., $L_{1 3}$ can be larger than $L_{2 3}$.
  In the conventional approach $L_{1 3} \approx L_{2 3}$ ($L_{1 2} \gg L_{2
3}$) and angle mixing of $\theta_{1 3}$ is very small. Angle mixings $\theta_{2
3}, \theta_{1 2}$ are big. Then there ia a problem: why is mixing angle
$\theta_{1 3}$ so small? A natural solution of the problem is to suppose that
$(m_2^2 - m_1^2) \neq (m_3^2 - m_1^2) - (m_3^2 - m_2^2)$, then $L_{1 3} > L_{2
3}$. It will be realized if there are 4 neutrino oscillations instead of 3
neutrino oscillations. Then the value of $\theta_{1 3}$ is necessary to search
at distances more than $L_{2 3}$.
