To find the value of \( c \) such that the expression \( x^2 + 4x + c \) is a perfect-square trinomial, we can use the formula for a perfect square trinomial, which states:
\[ (x + a)^2 = x^2 + 2ax + a^2 \]
Here, \( 2a \) corresponds to the coefficient of \( x \). In our expression \( x^2 + 4x + c \), the coefficient of \( x \) is 4, so we set:
\[ 2a = 4 \]
Solving for \( a \):
\[ a = \frac{4}{2} = 2 \]
Now, to find \( c \), we substitute \( a \) into the expression \( a^2 \):
\[ c = a^2 = 2^2 = 4 \]
Thus, the value of \( c \) is:
\[ \boxed{4} \]