  One of the most fascinating and technically demanding parts of the theory of
two-dimensional integrable systems constitute the models with the spectral
parameter on an elliptic curve, including Landau-Lifshitz and Krichever-Novikov
equations, as well as elliptic Toda and Ruijsenaars-Toda lattices, elliptic
Volterra lattice and Shabat-Yamilov lattice. We explain how all these models
can be unified on the basis of a single equation Q4 on a quad-graph. This
discrete model plays the role of the ``master equation'' in the (sl(2) part of)
2D integrability: most of other models in this area can be obtained from this
one by certain limiting procedures. More precisely, discrete versions of all of
the above mentioned models appear from Q4 imposed on a multi-dimensional cubic
lattice. Zero curvature representations for all models appear as a byproduct of
the general construction.
