= Tetraploidy
<Tetraploidy> is the presence of four complete <homologous chromosome> sets, conventionally written $4n$. It concerns chromosome-set number rather than simply how much <DNA> happens to be present. A diploid <cell> that has replicated its <DNA> before <mitosis> is normally still diploid, because each chromosome now has two sister chromatids rather than four independently inherited homologous sets. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others>[Chapter 18, Chromosome Duplication and Segregation: ploidy and sister chromatids]{ref}
This distinction matters in a DNA-content <histogram>. A tetraploid G1 <cell> and a diploid G2 <cell> can have the same total <DNA> signal despite different chromosome-set organization. Establishing <tetraploidy> may therefore require chromosome or <allele> information alongside bulk <DNA> content. Whole-genome duplication can create <tetraploidy>, while subsequent division and chromosome loss can change the descendants' <ploidy>.
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