  LHCb has recently reported the first observation of a four-body baryonic
$B\to{\bf B_1\bar B'_1 B_2\bar B'_2}$ decay, where ${\bf B_1\bar B'_1}$ and
${\bf B_2\bar B'_2}$ represent the two pairs of octet baryon states. In our
classification, the measured $\bar B^0\to p\bar p p\bar p$ decay is a tree
dominated process via internal $W$-boson emission, whose branching fraction is
explained as small as $2.2\times 10^{-8}$. We investigate for the first time
the phenomenology of other tree and penguin dominated $B\to{\bf B_1\bar B'_1
B_2\bar B'_2}$ decays, and predict the presence of a double threshold effect,
manifested as two peaks around $m_{\bf B_1\bar B'_1}\sim m_{\bf B_1}+m_{\bf\bar
B'_1}$ and $m_{\bf B_2\bar B'_2}\sim m_{\bf B_2}+m_{\bf\bar B'_2}$ in the
invariant mass spectra of ${\bf B_1\bar B'_1}$ and ${\bf B_2\bar B'_2}$,
respectively. Moreover, we predict the following branching fractions: ${\cal
B}(B^-\to n\bar p p\bar p)=(1.7^{+0.4}_{-0.2}\pm 0.1^{+0.7}_{-0.4})\times
10^{-7}$, ${\cal B}(B^-\to \Lambda\bar p p\bar p)=(7.4^{+0.6}_{-0.2}\pm
0.03^{+3.6}_{-2.6})\times 10^{-7}$, and ${\cal B}(\bar B^0_s\to \Lambda\bar
\Lambda p\bar p)=(1.9^{+0.3}_{-0.1}\pm 0.01^{+1.1}_{-0.6})\times 10^{-7}$,
which are accessible to experimental facilities.
