Ultra 4 (model A3579) from US - will it work in EU?

Yes, the Apple Watch Ultra 4 (Model A3579) bought in the US will work in the European Union and can connect to a Danish cellular plan. 

What to Keep in Mind

• Carrier Support: Ensure your specific Danish mobile carrier (such as TDC, Telenor, or 3) officially supports cellular service plans for the Apple Watch.
• Warranty Limitations: Apple Watch hardware warranties and AppleCare services may have regional servicing restrictions or require you to handle claims through the country of purchase or specific authorized guidelines, though standard Bluetooth, GPS, and Wi-Fi functionality will operate normally anywhere in the EU.

Thank you very much for your clear and helpful answer.

Just one follow-up question, as I want to be absolutely certain before purchasing the watch in the US:

Is your confirmation based on Apple’s official technical compatibility information, or are you aware of actual cases where a US-purchased Apple Watch Ultra 4 (A3579) has been successfully activated with a European carrier?

My main concern is whether a Danish carrier such as Telenor could reject eSIM activation specifically because the watch was purchased in the US, despite having the same model number as the European version.

Thanks again for your help!