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For Hamiltonian circle actions on 4-manifolds, we give a generators and relations description for the even part of the equivariant cohomology, as a module over the equivariant cohomology of a point. This description depends on combinatorial data encoded in the decorated graph of the manifold. We then give an explicit combinatorial description of all weak algebra isomorphisms. We use this description to prove that the even parts of the equivariant cohomology modules are weakly isomorphic (and the odd groups have the same ranks) if and only if the labelled graphs obtained from the decorated graphs by forgetting the height and area labels are isomorphic. As a consequence, we give an example of an isomorphism of equivariant cohomology modules that cannot be induced by an equivariant diffeomorphism of spaces preserving a compatible almost complex structure. We also deduce a soft proof that there are finitely many maximal Hamiltonian circle actions on a fixed closed symplectic 4-manifold., Comment: 77 pages, 15 figures. In v2, fixed a mistake in main example in Section 1 on the flavor of inequivalence; fixed a mistake in Section 6 where we now rely more heavily on component Euler classes; and myriad editorial changes throughout