Adding a revision term H 0 to the Baranowski-Bü ttner-Voit (BBV) model, we investigate the geometrical and electronic structure for an open chain with finite length of an emeraldine-base polymer. The numerical results are very similar to those in the Born-von Karman condition. On this basis, excitons and biexcitons are explored. Our solutions show that the exciton and biexciton in the period-4 EB polymer are not self-trapped like those in the LPPP or PNB, but form discrete polaron pairs and bipolaron pairs, respectively.