Step 1: Recall the formula for the common difference.
In an arithmetic progression (A.P.), the common difference is given by:
\[
d = a_2 - a_1
\]
Step 2: Substitute the values.
The first term \(a_1 = 2\) and the second term \(a_2 = 4\).
\[
d = 4 - 2 = 2
\]
Step 3: Conclusion.
Hence, the common difference of the given A.P. is 2. Final Answer:
\[
\boxed{d = 2}
\]