Why is the elevation on the GPS of my Vivoactive 4s inaccurate?

I had the same issue, probably associated with the last fw update, facrory reset solved the elevation (but not the increased battery discharge)

Thank you for your reply. I will do a reset. I have also made sure that autopause is on as I discovered it was off and apparently that can affect readings too.

My wife is also having problem with her VA4, showing usually -70 meters below sealevel. Before activity starts, she waits about 10 minutes during warm up.

I haven't experienced any other problems. I have changed to autopause on (on walks settings) and done a full reset (without losing data) - this is fiddly in itself. I am going out on the moors tomorrow so I will see if this rectifies the problem and report back.

Wife is using Lachesis (Light) watchface and Fat Burner datafield, will those be affected if doing a full reset? And how to do the full reset correct?

I had this issue a few firmware versions back. A reset to factory settings did the trick. Also reduced my devices battery consumption. 

And how to do the full reset correct? Yes, I can Google but since I am here... you get the picture.

I really don't know. I'm just a user myself. A full reset will be different on different watches. If on the vivoactive 4 or 4s, I imagine they are the same. Hold down bottom button until menu comes, scroll down to settings, then scroll down to system and reset is in there. On mine, it asked me whether I wanted a full reset losing my existing data or whether I wanted to retain my data. Good luck :-)

Sorry, Vivoactive 4S here. Full reset will delete everything in Garmin Connect?