The fourth aliyah begins with the words "these are the generations of Aharon and Moshe" and then goes on to discuss only the sons of Aharon. Rashi says that they are called the sons of Moshe because he taught them Torah.
Here, the tribe of Levi is appointed to take care of the Mishkan. Rashi says that since the Levites are doing this service as agents of all the Jewish people, the Jewish people give their tithe to the Levites. The Torah mentions that this honor originally belonged to all the first-borns, and Rashi says that because they participated in the chet ha-egel (sin of the golden calf) the first-borns lost the honor.
