Final answer:
Sigmund is likely to be in Freud's phallic stage of psychosexual development, experiencing desire for his mother and rivalry towards his father when observing their intimacy.
Step-by-step explanation:
Sigmund, the four-year-old, is likely to be in Freud's phallic stage of psychosexual development. This stage occurs between the ages of 3 and 6 and is characterized by the child becoming aware of their body and recognizing the differences between boys and girls. In this stage, the child experiences desire for the opposite-sex parent and jealousy towards the same-sex parent. For Sigmund, this would mean desiring his mother and feeling rivalry towards his father when observing their intimacy.