That problem wasn’t exactly easy, but the correct answer is 3. Here’s how we arrived at that answer.
Between 4 and 6, the difference is 2.
Between 6 and 10, the difference is 4.
Between 10 and 18, the difference is 8.
As you can see, the difference between the two numbers doubles as the problem progresses. Because of this, the number that comes before 4 is 3.