We consider in this work the fourth-order rational difference equation {Mathematical expression} where a ∈ [0, ∞) and the initial values x- 3, x- 2, x- 1, x0 ∈ (0, ∞). It is found that the perturbation of the initial values may lead to the variation of the trajectory structure rule for the solutions of the above equation. That is, with change of the initial values, the successive lengths of positive and negative semicycles for nontrivial solutions of this equation are found to occur periodically, and furthermore the periodicity is completel...