The Apple Watch Series 4 uses the T8006 (S4) Processor. It also contains a GPS module, and on cellular models an LTE chip.
Specifications
Display: LTPO OLED Retina
Screen: Ion-X glass or sapphire crystal
Compatibility (GPS): iOS 12 or later running on an iPhone 5s or newer
Compatibility (GPS+LTE): iOS 12 or later running on an iPhone 6 or newer