Three level-implicit finite difference methods of order four are discussed for the numerical solution of the mildly quasi-linear second-order hyperbolic equation A(x, t, u)u[subscript xx] + 2B(x, t, u)u[subscript xt] + C(x, t, u)u[subscript tt] = f(x, t, u, u[subscript x], u[subscript t]), 0 less than x less than 1, t greater than 0 subject to appropriate initial and Dirichlet boundary conditions. A new technique is introduced to obtain the stability range of the wave equation in polar coordinates. Fourth-order approximation at the first time level for a more general case is also discussed. The fourth-order accuracy of the method is demonstrated computationally by four examples. (Contains 4 tables.)