Building on the first three sutras, yoga sutra 1.4 states, Vritti sarupyam itaratra: When one is not in Self-realization, the Seer takes on the identity of the fluctuations of mind (thought patterns). That is, when we are wrapped up in our thoughts, unable to stay present, we become our thoughts.
This is most easily understood by considering an energetic action. Consider that when you become wrapped up in negative thoughts, those thoughts consume you. Your emotions take on a negative tone. Your words and actions often follow suit. You become the physical manifestation of those thoughts. Likewise, when you become wrapped up in happiness, your emotions, words, and actions follow. While you might think that becoming wrapped up in happiness is a goal, happiness, too, is finite. It comes and goes.
Instead, by not attaching to your mind chatter, you can effectively take a step back and observe them for what they really are—fluctuations. This practice can help you to notice your thoughts, and to notice when you are attaching to your thoughts and getting wrapped up in emotions, words, and actions. By becoming the Seer, or observing these patterns in yourself, you will become less affected by the day-to-day happenings in your life. You will be able to see them with more clarity.
Yoga sutra 1.4 tells us that when we are not in a state of yoga, the mind is attracted by the external world, the root of all suffering. It lays out the consequences of not practicing yoga, as defined in yoga sutra 1.2.