Article: Some 4-way switches may have the "in" and "out" pairs opposite each other, rather than top and bottom. Look at the markings on yours and read the instructions carefully. If you are adding more 4-way switches, repeat these steps with the two 3-conductor cables entering each 4-way switch location from the adjacent switches.

What is a summary?
Attach the red wire entering the 4-way switch box from the first 3-way switch box to the red wire going to the next 4-way switch. Attach the black and white/blue traveler wires entering the 4-way switch box to the "in" terminals, often the top terminals on the 4-way switch - black on the left top pole and white/blue on the right top pole. Attach the black and white/blue traveler wires exiting the 4-way switch box to the next 4-way (or final 3-way) switch box to the "out" terminals, often the bottom terminals on the 4-way switch - black on the left bottom pole and white/blue on the right bottom pole.