Thanks for the question. It is new for you that WhatsApp can be used on Garmin Venu 4. As I understand it, WhatsApp on Garmin shows the last 10 chats/messages. That's as much as I have right now. Sorry I can't help you better.
The manufacturer sells the watch in its own shop at the official list price. Dealers like us can often offer more favourable prices due to purchasing conditions, promotions and competition.
No, you cannot activate Strava directly from the watch. Integrations with Strava are done via the Garmin Connect app on your phone or website, synchronising activities and routes after accounts are linked.
The watch cannot record an ECG during activity. The ECG app records the electrical signals of the heartbeat to detect signs of atrial fibrillation, but the ECG measurement takes place as a separate, approximately 2-minute session and not continuously during exercise.
You cannot import GPX files directly to the Garmin Venu 4. Instead, you can manage the routes via the Garmin Connect app or other third-party apps and transfer them to the watch. The Swisstopo app itself may not support direct transfer to Garmin devices, but you can export the routes in GPX format and then import them via Garmin Connect.
The strap is exactly as pictured. Although it says in the description that it is black, in reality when I opened it from the box I saw that it was grey. But it's actually even nicer than the black one for me.