Universal four-body physics. The lower panel illustrates the extension of the Efimov’s scenario of Fig. 1 to four identical bosons. The red solid lines illustrate the pairs of universal tetramer states associated with each Efimov trimer. In the four-body picture, the Efimov trimers give rise to the trimer-atom thresholds (green dashed lines) and there also appears a dimer-dimer continuum (orange). The two arrows indicate the positions where the pair of four-body states couples to the atomic threshold. The upper panel shows the experimental results  on atomic four-body recombination as a function of the scattering length a. The resonant increase observed at around -410 a0 is induced by one of the predicted universal tetramer state intersecting the four-atom threshold .