A possible solution to resolving a mamzer problem is debated in the mishna based on a halacha learned in the previous mishna about the offspring of a Caananaite maidservant. Rabbi Tarfon explains that if the offspring goes by the mother, then a male mamzer can marry her and have the master free the offspring in which case the offspring would be Jewish and not a mamzer. Rabbi Eliezer disagrees. The Gemara explains that Rabbi Tarfon allows this ab initio. The fourth chapter begins with a description of all the different lineages of the people who came to Israel in the time of Ezra and who were permitted to marry whom. There is a debate between Rava and Abaye about what happened historically.  Did Ezra forcibly remove all those with problematic lineage when he went to Israel to ensure that proper records would be kept and people wouldn't marry in forbidden marriages or did he recommend it and most of them followed of their own free will?