Graph bootstrap percolation is a cellular automaton introduced by Bollobas. Let H be a graph. Edges are added to an initial graph G=(V,E) if they are in a copy of H minus an edge, until no further edges can be added. If eventually the complete graph on V is obtained, G is said to H-percolate. We identify the sharp threshold for K4-percolation on the Erdos-Renyi graph G(n,p)...