For the equilibrium reaction:
\[
N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)
\]
At a certain temperature, the equilibrium constant \( K_c \) is 4.0. If the concentrations of \( N_2 \), \( H_2 \), and \( NH_3 \) are 0.2 M, 0.6 M, and 0.4 M, respectively, calculate the reaction quotient \( Q_c \) and determine whether the reaction is at equilibrium.