In this section, Freud reviews three different theories on the sources of dreams: one arguing that dreams come from external sensory stimuli; one arguing that dreams come from internal sensory stimuli; and another arguing that dreams come from "purely psychical sources of stimulation" (1.4.3).
Freud discusses the legitimacy of each of these theories. After reviewing the first, he concedes that some elements of dreams seem to come from external stimuli—but not all. The same goes for internal sensory stimuli, too.
Freud moves on to discuss the psychical sources of dreams and argues that the "enigma" of where dreams come from "can be solved by the revelation of an unsuspected source of stimulation" (1.4.59). That's big news, but he doesn't reveal what that "unsuspected source of stimulation" is.