I am bit confused with a statement I read. It talks about $H\to 4\ell$ decay width (higgs to 4 lepton). Now, higgs can decay through different modes. But, as far as I know it doesn't decay directly to 4 leptons (atleast not at leading order).But anyways, it can also decay (generally) to Z Z* which then decay to 4 leptons. So, my question is what does h->4l decay width means? I should add the decay width of the intermediate decay process of Z to dileptons to the decay width to Higgs to Z Z boson to get h->4l decay width?
I general, one needs to compute the matrix elements of the whole diagram: the first one I have drawn in that answer to another question, then sum it with the matrix element for the second one, and any other relevant ones. Indeed $Z$ bosons are typically off-shell when the invariant mass of the final state is above twice the mass of $Z$. So, no, you can't just add decay widths.