In the recursively defined arithmetic sequence you provided, the first term is \( a_1 = 4 \) and the recursively defined formula is \( a_n = a_{n-1} - 5 \).
The common difference \( d \) in an arithmetic sequence is the constant amount that each term differs from the previous term.
From the recursive formula \( a_n = a_{n-1} - 5 \), we can see that each term is obtained by subtracting 5 from the previous term. Therefore, the common difference is:
\[ d = -5 \]
So, the common difference is \(-5\).