The correct order of the steps to prove that the sum of two rational numbers \(a\) and \(b\) is rational is as follows:
- Step 3: Write \(a\) and \(b\) as fractions.
- Step 2: Find a common denominator.
- Step 4: Add the fractions.
- Step 1: Identify the sum as rational.
Therefore, the answer is: Step 3, Step 2, Step 4, Step 1.