Using two devices to the same Garmin Connect App

Basically yes, it works, but you may experience diverse problems, especially if the devices are not of the same generation. Also, the Primary Wearable Device will always take priority for some of the metrics. Some info is available at Can I Pair More Than One Device to the Garmin Connect App? | Garmin Customer Support

In reality I would like to use the smart bracelet, in this case the vívofit 4 which I have not yet purchased, as a replacement for the Fenix 6 when I want to wear another classic watch on my wrist, so that parameters such as heart rate, steps, floors climbed etc are still measured. Do you think it would work?

At the beginning I thought of using a smart ring, at the moment there are none compatible with Garmin

I have personally no experience with multiple devices used on the same account, just see some problems reported here on the forum. Depending on the patterns of wearing the watches, you may need to change the Primary Wearable Device in the settings each time you swap them. Though it may not be necessary for the basic metrics, and I guess you will not record any activities with the vivofit 4, so perhaps it could work even without the permanent settings changes.

However, I'd tell that using your Fenix 6 attached to the ankle, when you want to wear a classical watch, is a much simpler, cheaper, and safer solution. Yes, it does measure the HR (and other metrics) even on the ankle.

Alternatively you can also consider using the HRM-Pro chest strap that can record metrics like HR, steps, Active Calories, or Intensity Minutes too, and additionally it is a great enhancement of the watch for most sports.