Tetraploidy is the presence of four complete homologous chromosome sets, conventionally written . 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. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 18, Chromosome Duplication and Segregation: ploidy and sister chromatids
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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