The best approach for modeling city assets would be to create individual
modular assets, that can be reconstructed in UE4.
Personally, I would start by setting up Blender/Cinema 4D to the correct UE4 scale,
then block out the basic structure of the level using bsp brushes in UE4. (this is a
general starting point for most medium to large scale enviroments)
This assures two things, the correct scaling and how much work is needed to be put
into the level. This of course has its advantages and disadvantages, for example; it's
easier to texture smaller module pieces than larger objects (a texture atlas would be used for larger buildings in the distance),
module pieces can also cause light bleeding (due to incorrect lightmaps), this could be solved by
creating a second lightmap channel and not overwriting it in UE4.
Module assets can also be easily recognised by the player (if done incorrectly) this is
most commonly caused by no variance in textures or meshes. Decals work great for
covering up mistakes or tiling meshes/textures. A second aproach would be
to create procedural buildings, this requires more work but eventually pays off in the
final scene. Good luck.