In watchOS 3 the 'Now Playing' complication played music on your phone when you pressed play, unless you changed audio source to the watch.
In watchOS 4 this defaults to the watch, and so far as I can tell there's no way to change it to the phone.
In practice this means that I will travel somewhere listening to music, arrive at my destination, do something else for a bit, then when leaving I press 'Play' on my watch and it starts a completely different album or playlist from the watch instead of continuing where the iPhone left off (the desired behaviour – and the default if I pressed 'Play' on the iPhone, or in watchOS 3).
Since it's not an LTE watch, and it has very little storage on it for music, this change seems completely bizarre as it means that there's only a small selection of tracks to play ever.
My question: is there a way to disable this, and make the Watch start music via the iPhone instead of the Watch?