I had cataract surgery 4 days ago and I am seeing a streak of light when I look directly at a light source. Is this normal?

Astigmatism. It sounds like you have some astigmatism. It could be temporary and improve with healing. If your vision otherwise seems good, i would not be concerned but ask your doctor about it at your next follow-up visit.
It may be normal. There are several visual phenomena that people may experience shortly after cataract surgery: streaks of light, shimmering, shadows, halos, arcs of light to name a few. These tend to dissipate after 1-2 weeks. Any new phenomena should be evaluated by your surgeon, as should be loss or decrease in your vision since surgery. Otherwise, you should continue to improve over the next few days.
Maybe. It is common to see a light or dark semicircle temporally after cataract surgery. This is due to the lens implant and typically resolves as the lens heals in place. You should see your eye doctor to be sure that it is not something more significant.
Streak of light! If you mean a steak of light when looking at lights, like a line of light extending off of the light source both directions, it is most likely a fold in the posterior capsule, and will either settle on its own, or many need yag laser at a later date (a minimal procedure). If you are seeing flickers or flashes of light, this could be part of the healing process or vitreous changes, tell your md.
A wrinkle maybe. Agree that this could be astigmatism, but more likely the capsular bag (membrane in which the implant is placed) is stretched during healing, causing a wrinkle, which would thus cause a beam of light perpendicular to that wrinkle. Should resolve spontaneously. Sometimes a yag laser is needed to cure if persists.