A circular saw is much better than a small table saw for cutting 4x8 sheets. Make a jig for cutting straight lines with your circular saw by nailing a 1x4 to a piece of plywood. Then rest the saw up against the 1x4 and cut the plywood. To use this jig, just put the plywood edge on the line you want to cut and clamp it down. Using the 1x4 as a guide, cut away.