This just happened to me with my Vivoactive 4s. I hit my 11,000 step goal and exceeded it only later to see my Garmin display 5,000. I tried powering it off and on, syncing it. Nothing worked.
When you write "my Gamin", do you mean your watch, or the phone app Garmin Connect Mobile? If it was the watch that lost the data, then I would contact the support, or post in the forum for Vivoactive 4. If it was the app, then I would try clearing the app cache in Android Setting » Apps » Connect, and then rebooting the phone.
BTW, when the problem occurs on your watch, the disucssion does not belong onto the Garmin Connect Mobile forum. You'd better post about the issue on the respective forum dedicated to your model of watch, where other users of the same model may help you better.