A full game can have as many polygons as you want really, the limitation does not apply in the sense of total poly’s per game. What you need to be concerned with is the number of poly’s being drawn on screen at any time during the game.
For characters the recommendation for the last gen titles was to keep them below 30k tri’s, this is still a good recommendation, but you can go higher than this if required.
To keep the poly count down make sure you are baking as much information onto the low poly mesh as possible, using normal maps created based on your high poly mesh to fill in the smaller details, allowing you to use less poly’s for (almost) the same result. A lot of fine detail can be added through through the material, it is not always necessary to sculpt it onto the mesh.
Basically for all meshes you create try and reduce the poly count as far as possible while still retaining the detail you need. Use LOD’s and normal maps wherever possible to add in extra detail as it is required.
You can create a map like BF4 in UE4 as well, the maximum size is actually quite a bit larger than the map in that game. The details of the map and models that populate it will need to be created, but there is no limitation that would prevent you from creating a game of that scope. It will be very difficult without a large team, but technically it is possible given enough time.