To prove that the sum of two rational numbers \( a \) and \( b \) is rational, you can put the steps in the following order:
- Step 3: Write \( a \) and \( b \) as fractions.
- Step 2: Find the common denominator.
- Step 4: Add the fractions.
- Step 1: Identify the sum as rational.
So the correct order is: 3, 2, 4, 1.