To find the fourth term of the sequence defined by the formula \( a_n = 8n - 11 \), we need to substitute \( n = 4 \) into the formula.
\[ a_4 = 8(4) - 11 \] \[ a_4 = 32 - 11 \] \[ a_4 = 21 \]
Therefore, the fourth term of the sequence is \( 21 \).