To determine the height at which the tennis ball is hit, we need to evaluate the given equation \( h = -16t^2 + 18t + 4 \) at the moment it is hit, which is at \( t = 0 \) seconds.
Substituting \( t = 0 \) into the equation:
\[ h = -16(0)^2 + 18(0) + 4 \] \[ h = 0 + 0 + 4 \] \[ h = 4 \]
Therefore, the height at which the tennis ball is hit is 4 feet.