The given sequence is: 4, 5, 6, 7, 8, ...
This is an arithmetic sequence where the first term \( a_1 = 4 \) and the common difference \( d = 1 \).
The explicit formula for the \( n \)-th term of an arithmetic sequence can be expressed as:
\[ a_n = a_1 + (n - 1)d \]
Substituting the values into the formula:
\[ a_n = 4 + (n - 1) \cdot 1 \]
Simplifying this:
\[ a_n = 4 + n - 1 = n + 3 \]
So the explicit formula for the given sequence is:
\[ a_n = n + 3 \]
Now, to find the fifteenth term (\( n = 15 \)):
\[ a_{15} = 15 + 3 = 18 \]
Thus, the fifteenth term is:
\[ \boxed{18} \]