4-way pipe fitting is also called pipe cross. If a branch line passes completely through a pipe tee, the fitting becomes a cross. A cross has one inlet and three outlets, or vice versa. They often have solvent welded socket ends or female threaded ends.
4-way pipe fitting can generate a huge amount of stress on pipe as temperature changes, because they are at the center of four connection points. A tee is more steady than a cross, as a tee behaves like a three-legged stool, while a cross behaves like a four-legged stool. (Geometrically, “any 3 non-colinear points define a plane” thus 3 legs are inherently stable.) Crosses are common in fire sprinkler systems, but not in plumbing, due to their extra cost as compared to using two tees.
Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)