Go to Settings → System → About My System. Here you’ll see the Mac addresses of the players. WM: 0 shows players that are wired, WM: 1 shows players that use WiFi. ROAM will always be WM: 1.
Is this extra device shown in Settings? I’ve never seen the behavior that you describe, however, these devices do have 2.4 and 5GHz radios and they might switch access points, particularly the ROAM’s if you move them around. It’s possible that your WiFi manager noticed another radio and logged this, but I doubt that all four devices will be online simultaneously. However, if your manager hangs on to recent connections there may be some overlap before stale data is removed from view. If a unit is being double counted, I expect that the Mac addresses will be different only by a couple characters at the end of the string.