Pixel Watch 4 owners can install five Pixel Watch 5 watch faces by sideloading Pixel Watch Faces app version 5.0.231.924930285 through GeminiMan WearOS Manager.
The workaround requires Developer Options, ADB and Wireless Debugging, then pairing the watch and phone on the same Wi-Fi network to push the APK over wireless ADB.
The sideload adds Weave, Chronograph, Steph Curry-edition, Multicolor Reveal and Multicolor Modular faces, extending some of the Pixel Watch 5 software extras to last year's model.
Pixel Watch 1, 2 and 3 are not supported, likely because the newer faces depend on Watch Face Format requirements that those models cannot load.