A face always has only one normal with one direction, there’s no inside or outside. As soon as a model is made up of multiple faces, the terms inside/outside begin making sense as with the cube, which has an inside and an outside.
What you can do in UE4 is using a Two Sided Material (there’s a checkbox in material editor). In this case, both sides of a triangle will receive the material and the triangle will be visible from both sides. Maybe this is what you want to achieve.
Here’s some more example from blender to clarify:
Left: normals pointing outside, starting at the dot in the middle of each face. This will render fine in UE4.
Middle: normals changed by recalculate normals - this makes all normals pointing inside now - that is a bad case for UE4 import, if the cube should be rendered correctly.
Right: that’s the “special” case with flat object, I mentioned above. The normals might look as if they are pointing outside, but actually they do not and it is just the inside normal of one face coming out at the opposite face. You need to zoom in and check, where each normal acually starts and in which direction it points. Tip: be sure to be in Face select mode so that you can see the black dot from where normals are drawn, so you can see the origin. It’s also useful to shorten the length to a lower value than the distance of the faces to better see, if normals are pointing to the outside of the model.
