I don't understand why in all tutorials on cell mitosis the diagrams show 4 pairs of sister chromatids instead of two? I know that before mitosis each pair of homologous chromosomes is duplicated into sister chromatids. In that case why don't they just put 2 pairs of sister chromatids (1 from the maternal homologue, 1 from the paternal homologue) in the diagrams instead of 4?