Four-body baryonic $B\to {\mathbf{B}}_{\mathbf{1}}{\bar{\mathbf{B}}}_{\mathbf{1}}^{\mathbf{\prime }}{\mathbf{B}}_{\mathbf{2}}{\bar{\mathbf{B}}}_{\mathbf{2}}^{\mathbf{\prime }}$ decays

LHCb has recently reported the first observation of a four-body baryonic $B\to {\mathbf{B}}_{\mathbf{1}}{\bar{\mathbf{B}}}_{\mathbf{1}}^{\mathbf{\prime }}{\mathbf{B}}_{\mathbf{2}}{\bar{\mathbf{B}}}_{\mathbf{2}}^{\mathbf{\prime }}$ decay, where ${\mathbf{B}}_{\mathbf{1}}{\bar{\mathbf{B}}}_{\mathbf{1}}^{\mathbf{\prime }}$ and ${\mathbf{B}}_{\mathbf{2}}{\bar{\mathbf{B}}}_{\mathbf{2}}^{\mathbf{\prime }}$ 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 $B\to {\mathbf{B}}_{\mathbf{1}}{\bar{\mathbf{B}}}_{\mathbf{1}}^{\mathbf{\prime }}{\mathbf{B}}_{\mathbf{2}}{\bar{\mathbf{B}}}_{\mathbf{2}}^{\mathbf{\prime }}$ decays, and predict the presence of a double threshold effect, manifested as two peaks around $m_{{\mathbf{B}}_{\mathbf{1}}{\bar{\mathbf{B}}}_{\mathbf{1}}^{\mathbf{\prime }}}\sim m_{{\mathbf{B}}_{\mathbf{1}}}+m_{{\bar{\mathbf{B}}}_{\mathbf{1}}^{\mathbf{\prime }}}$ and $m_{{\mathbf{B}}_{\mathbf{2}}{\bar{\mathbf{B}}}_{\mathbf{2}}^{\mathbf{\prime }}}\sim m_{{\mathbf{B}}_{\mathbf{2}}}+m_{{\bar{\mathbf{B}}}_{\mathbf{2}}^{\mathbf{\prime }}}$ in the invariant mass spectra of ${\mathbf{B}}_{\mathbf{1}}{\bar{\mathbf{B}}}_{\mathbf{1}}^{\mathbf{\prime }}$ and ${\mathbf{B}}_{\mathbf{2}}{\bar{\mathbf{B}}}_{\mathbf{2}}^{\mathbf{\prime }}$, respectively. Moreover, we predict the following branching fractions: $\mathcal{B}(B^{-}\to n\bar{p}p\bar{p})=(1.7_{-0.2}^{+0.4}\pm 0.1_{-0.4}^{+0.7})\times {10}^{-7}$, $\mathcal{B}(B^{-}\to \mathrm{\Lambda }\bar{p}p\bar{p})=(7.4_{-0.2}^{+0.6}\pm 0.03_{-2.6}^{+3.6})\times {10}^{-7}$, and $\mathcal{B}({\bar{B}}_s^0\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}p\bar{p})=(1.9_{-0.1}^{+0.3}\pm 0.01_{-0.6}^{+1.1})\times {10}^{-7}$, which are accessible to experimental facilities.