The fourth aliyah begins a set of commandments that describe the whole set of Yom Tovim of the year, and this discussion will continue for the next few aliyahs. The discussion begins with the commandment to observe the Shabbos. Rashi asks what does the Shabbos have to do with the festivals? He answers that their proximity here teaches that whoever desecrates a festival is as if he desecrates the Shabbos, but whoever observes the festivals, is credited as if he observed the Shabbos.
This aliyah then describes the holiday of Pesach, then the counting of the omer (which we are in right now), then the holiday of Shavous. Note that Shavous is unique in not being on a fixed calendar date like Pesach (which always begins the evening of the fourteenth of Nissan) and Sukkos (always the fifteenth of Tishrei), as well as Rosh Hashanah (first and second of Tishrei), and Yom Kippur (the tenth of Tishrei). Instead, Shavous is determined by counting 49 days from Pesach. This is important to know, especially if one is planning on traveling over the international dateline.
