I'm just letting others know how my Vivoactive 4s charging problem were fixed:
For several months, my Vivoactive 4s had sporadic charging issues. Sometimes it didn't charge after plugging it in. It slowly became worse until finally it wouldn't charge at all.
To be honest I was frustrated with the product. I thought surely this must be a hardware issue with the watch. I was ready to ditch Garmin products for the Apple Watch, despite being a loyal Garmin watch customer for a long time.
Even though my watch was out of warranty, I gave Garmin Support a call. They had me take a photo of the charging cable. They immediately diagnosed it as the problem.
All 4 of the pins in the charging cable are supposed to be of equal length. 2 of my pins (one outer, one middle) were ground down or stuck. They replaced the charging cable out of courtesy and now it works!
I'm a very happy customer now. I don't know how the charging pins got worn down but this is something to rule out if you have charging issues.