Final answer:
The unusual structure with four pairs of sister chromatids formed during meiosis I is called a synaptonemal complex, which facilitates crossing over and forms a tetrad.
Step-by-step explanation:
The unusual structure containing four pairs of sister chromatids that you are referring to is known as a synaptonemal complex during early prophase I of meiosis. When homologous chromosomes come into tight pairing, or synapsis, they are aligned precisely by this protein lattice, and crossing over occurs between non-sister homologous chromatids at locations called recombination nodules. This process is visible as chiasmata (singular = chiasma). As prophase I progresses, the synaptonemal complex begins to break down, yet the chromosomes remain bound together at the centromeres and chiasmata, now forming a tetrad. This ensures the proper separation of homologous chromosomes during meiosis I.
The unusual structure formed during meiosis I in an individual carrying a reciprocal translocation is called a tetrad. A tetrad consists of two duplicated homologous chromosomes, each with two sister chromatids, bound together by chiasmata. During prophase I, the synaptonemal complex and recombination nodules facilitate the alignment and exchange of genetic material between non-sister homologous chromatids, resulting in the formation of chiasmata and tetrads.