The fourth aliyah describes the Divine signs that signal that the Jews should move their camp. From the day the Mishkan was erected a cloud covered the Mishkan by day and a fire by night. Whenever the cloud lifted up they would journey, and whenever it rested they would stop. In some cases the cloud rested for just a few days, or even one night, and at other times the cloud rested a whole year. In all cases the Jewish people trusted in G-d and moved at His bidding.
Next, this aliyah discusses the two silver trumpets Moshe is to make and use. Aharon's sons would blow the trumpets at Moshe's command. Different trumpet blasts meant different things; including assembling the community together for a meeting, or just the princes, or that the camp was to start moving. And when we went to war, a certain trumpet blast would cause us to be remembered by G-d, and then we were delivered from our enemies.
