Concentration of A- in Weak Acid Solution

In a certain biochemical pathway, a weak acid (HA) dissociates according to the equilibrium:

HA (aq) <=> H⁺ (aq) + A^- (aq)

Given the equilibrium constant (K_a) for the dissociation of this acid at 25°C is 5.0 x 10^-5. If the solution is prepared such that the initial concentration of HA is 0.10 M and the pH of the solution is measured to be 4.00 after some time:

Calculate the concentration of the A^- ions at equilibrium. Which of the following statements best describes the concentration of A^- in this weak acid solution?