In the recursively defined arithmetic sequence given by:
- \( a_1 = 4 \)
- \( a_n = a_{n-1} - 5 \)
The common difference \( d \) can be identified as the amount you subtract to get from one term to the next.
From the definition, each term is formed by subtracting 5 from the previous term. This means that the common difference is:
\[ d = -5 \]
Thus, the common difference in this arithmetic sequence is \( -5 \).