Unlike Apple Watch, where you present the face for NFC payment, the Galaxy Watch 4 seems to require something else....

This may help others diagnose the problem. I saw a comment somewhere about using an Android app called NFC Reader. It has an option to "Scan a Tag", which, once tapped, will display NFC information about the device/card presented to it. That seems to work with both the bottom and top edges for the GW4. I've noticed that sometimes I have to continue to hold the watch to a reader in a shop and the phone buzzes twice: the first attempt is rejected and the second accepted. I just did the same with NFC Reader app. The first attempt shows a lot of strange characters but the second is parsed correctly by the NFC Reader app. The GW4 NFC technology is shown to be "IsoDep, NfcB". I assume there are different signal types/standard? That may explain why some readers say present one card only when I use the watch to make payment. From searching, this seems to be a firmware fault in those readers. My guess is the watch is sending two NFC signals to readers to allow for different standards?