In chapter 11 of Fourth Wing, Violet has a hard time with the obstacle that prepares them to mount a dragon. This foreshadows her inability mount and hold seat after she bonds with Tairn.
In chapter 12 of Fourth Wing, during Presentation Luca notes that Andarna (known as “the feathertail” at that point) is “…not even powerful enough to be a real color” (Yarros 148). This foreshadows a huge plot point towards the end of Iron Flame, where we learn that Andarna doesn’t have a true color.
In chapter 16 of Fourth Wing right after Threshing, Xaden tells Violet that she “…put[s] too much faith in [Dain]…And knowing who to trust is the only thing that will keep [her] alive–keep [them] alive–not only in the quadrant but after graduation” (Yarros 188). This foreshadows one of the large plot points towards the end of the novel where Dain betrays Violet’s trust by telling his father what he saw in Violet’s recent memories, putting her life in danger.
In chapter 19 of Fourth Wing after Violet is jumped in her dorm, Xaden storms to the flight field with her to demand answers about how time stopped. He speaks to Tairn and Sgaeyl, and this is where Violet finds out that due to Tairn and Sgaeyl’s bond, she’s able to communicate with Sgaeyl. Xaden can also communicate with Tairn. Xaden claims that he “…need[s] to know what happened in that room” (Yarros 232). Tairn replies “Do not dare to try and read me, human, or you’ll regret it” (Yarros 232). This heavily hints at and foreshadows Xaden’s second signet that’s revealed towards the end of Iron Flame.
In chapter 19 of Fourth Wing Xaden tells Violet: “Rare abilities, when kept secret, are the most valuable form of currency we possess” (Yarros 236). This foreshadows Xaden’s second signet and his reasons for keeping it a secret.