The correct sequence of steps for the proof that the sum \( t = r + s \) is irrational if \( r \) is rational and \( s \) is irrational would be as follows:
1. **Step 2**: For a contradiction, assume that \( t \) is rational, and write \( r \) and \( t \) as fractions.
2. **Step 1**: Subtract, writing \( s \) as a fraction. Since \( r \) is rational, we can express \( t \) as \( t = r + s \) and hence \( s = t - r \).
3. **Step 4**: Find a common denominator for the fractions representing \( r \) and \( t \).
4. **Step 3**: Realize a contradiction, because if both \( r \) and \( t \) are rational, then \( s \) would also have to be rational, which contradicts the assumption that \( s \) is irrational.
So the correct order is: **2, 1, 4, 3.**