The fourth aliyah contains the end of parsha Acharei and the beginning of parsha Kedoshim and discusses the prohibition of homosexuality and of bestiality of men and woman. G-d says that the Jewish people shouldn't defile themselves with these things as the nations being cast out before them in the land of Canaan defiled the land with these actions. The Torah adds that that's why the land vomited them out. Rashi says that just as a person can't keep vile food in his stomach, the land of Israel can't contain transgressors.
Parsha Kedoshim then begins. G-d commands Moshe to admonish the Jewish people to be holy, then summarizes several laws, such as: fearing parents, observing Shabbos, leaving over parts of the harvested field for the poor, not to lie, not to put a stumbling block before the blind, and others.
