I want to get into Aerospace Engineering. Is a 4-year degree enough to get started?

Just barely, it will get you in on the ground floor but you will need the masters degree to go further.

Yes, a 4 year degree in aerospace engineering should be a good start to a career. Try to get some Human Factors training along the way. Internships are always good to give you some experience. Internships can also help you to see different aspects of working in the Aerospace industry, and might help you to decide what direction you want to go in your career.

In your high school or 12th grade, you should be a top student in Physics and Maths.
Aerospace Engineering is highly specialized field, meaning that after Bachelor Degree in Aerospace, you will
concentrate in single field e.g. Aerodynamics, or robotics, or flight control mechanics, propulsion systems etc.
Aerospace engineers ar experts in MechNics, Thermodynamics, Robotics, or Aerodynamics.
Four year degree in Aerospace Engineering will make you qualified to work in any of the above field with any top defense aerospace company.
Masters degree is required if you want to do research or some sort of teaching at Aerospace School or university.
Previously "Aeronautical Engineering" Terminology was used for the above engineering skills but as the filed
advanced into outer space, Aerospace Engineering Term was introduced and now it covers two fields,
e.g. Aeronautical Engineering with focus on Aircraft and Astronautical Engineering with focus on Space Craft
It is very exciting field and I wish I had one more life to do all this one more time. But you should research on job prospects before deciding to jump into it.

Absolutely yes. In fact, there are Liason engineers (in the U.S.) with no formal education.
A 4 year degree will set you on a great career path.