Four-way tees are used for pipe fittings and pipe connections. Also known as four-way pipe fittings or four-way tees, they are used at the junction where the main pipe branches off. Four-way tees come in both equal and unequal sizes; equal-size four-way tees have the same size at the outlet ends; unequal-size four-way tees have the main pipe outlet with the same size, while the branch pipe outlet is smaller. Four-way tees are pipe fittings used at the branching points of pipes. The hydraulic expansion forming process of a four-way tee involves using a hydraulic press to inject liquid into a pipe blank of the same diameter as the four-way tee, then synchronously squeezing the blank with the two horizontal cylinders of the hydraulic press. As the blank is compressed, its volume decreases, and the liquid inside the blank increases in pressure. When the pressure reaches the required level for the branch pipe to expand, the metal material flows and expands into the branch pipe under the combined pressure from the side cylinder and the liquid pressure inside the blank.