  We continue our previous determination of the masses of the sequential fourth
generation quarks in an extension of the Standard Model, and predict the mass
$m_4$ ($m_L$) of the fourth generation neutrino $\nu_4$ (charged lepton $L$) by
solving the dispersion relation associated with heavy fermion decays. The
results $m_4\approx 170$ GeV and $m_L\approx 270$ GeVare extracted from the
dispersive analyses of the $t\to d e^+\nu_4$ and $L^-\to \nu_1 \bar t d$ decay
widths, respectively, where $t$ ($d$, $e^+$, $\nu_1$) denotes a top quark (down
quark, positron, light neutrino). The predictions are then cross-checked by
examining the $L^-\to \nu_4 \bar u d$ decay, $\bar u$ being an anti-up quark.
It is shown that the fourth generation leptons with the above masses survive
the current experimental bounds from Higgs boson decays into photon pairs and
from the oblique parameters. We also revisit how the existence of the fourth
generation leptons impacts the dispersive constraints on the neutrino masses
and the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix elements. It is found
that the unitarity of the $3\times 3$ PMNS matrix holds well up to corrections
of $O(m_\nu^2/m_W^2)$, $m_\nu$ ($m_W$) being a light neutrino (the $W$ boson)
mass, whose mixing angles and $CP$ phase prefer the values $\theta_{12}\approx
34^\circ$, $\theta_{23}\approx 46^\circ$, $\theta_{13}\approx 4^\circ$ and
$\delta\approx 235^\circ$ in the normal-ordering scenario for neutrino masses.
On the contrary, the unitarity of the $3\times 3$ Cabibbo-Kobayashi-Maskawa
matrix is violated at the level $m_b^2/m_W^2\sim 10^{-3}$, $m_b$ being the $b$
quark mass, as the same formalism is applied to the quark mixing.
