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We study the gluon fusion into pairs of o -shell Z bosons and their subsequent decay into charged lepton pairs at hadron colliders : gg ! ZZ ! 4` (` : charged lepton) .Throughout this paper we do not restrict the intermediate state Z bosons to the narrow width approximation but allow for arbitrary invariant masses. We compare the strength of this process with the known leading order results for q q ! ZZ ! 4` and for gg ! H ! ZZ ! 4` . At LHC energies ( p s = 14 TeV) the contribution from the gluon fusion background is around 20 % of the contribution from quark-antiquark annihilation. These two processes do not form a severe irreducible background to the Higgs signal. At Higgs masses below 120 GeV the nal state interference for the decay channel H ! ZZ ! 4 is increasingly constructive. This has no e ect on the Higgs search as in this mass region the signal remains too small. One can extend the intermediate mass Higgs search via o -shell Z boson pairs at the LHC down to about 130 GeV Higgs mass. However a careful study of the reducible background is needed for de nitive conclusions.