Why didn’t my v4 record my open water swim this morning??

It doesn’t. It loses gps when it goes more than a few inches underwater. What I do is tow an inflatable behind me when I swim, and put the watch inside the inflatable with my towel and keys. The more expensive watches do track open water swims correctly I believe.

3295023 said:

I was under the impression it records swimmIng automatically

The vivoactive 4 doesn't record any type of swimming automatically, only walking and running. Also, there is no OWS option even manually it would be just pool swimming - vívosmart 4 - Recording a Swim Activity (garmin.com)

If you want a GPS track of the activity you could start it as "Other" or "cycling," but it would be sketchy at best as mentioned by 6104177. Then change the activity type to Open Water Swimming in Garmin Connect.

You can install an app on the Vivoactive 4 for tracking OWS. I did it. It works as long as you keep your watch arm out of the water.