I also have maps that include lots of exterior environments, which I just have a hard time convincing myself to do without, so it creates problems. I have had to do a lot of optimization to get my game to play with a higher fps rate. I love modular construction, but the truth is after 1000 object calls it just all starts to fall apart so I do the following.
1 ) After I build an environment I look for groupings of objects. I can export that grouping to Maya as an fbx file from UE4, then template it and make my groupings in Maya instead. If i can save between five to ten object calls into a grouping that’s a good savings especially if it’s used twenty times in an environment. The trick is balancing between making the grouping too big, so that you start getting errors and shadow problems that requite the shadow minimum render to get larger and larger. Sometimes I err on the side of grouping ten objects together and see what I get, only to go back and slim it down.
2 ) Make LOD objects for your model. These should be reduced in your modeler of choice because the lightMap UVs need to match. There may be other ways to do this, but from my experience if your lightMap UVs change, you will get lots of wonky shadowy things happening when the game switches to lower polygon models.
3 ) I make smaller texture maps regardless of even having a LOD model. So I copy my original model into a MRES (medium res) and an LRES (Low res ) and even an XLRES (extra low). This is the technique I used in VFX, and then I would run all my 2k texture maps through a filter to reduce them to 1k, 512, 256 and 128 resolution. You can do this manually in Photoshop.
When you create your LOD1 and LOD2 in UE4, make lower resolution Materials to go on your LOD1 and LOD2 models using the reduced textures you did as above. This will at least cull out loads of texture data. If you have LOD models all the better.
4 ) I use Streaming Levels to bring in areas of models (especially interiors and interior props - anything I can hide) and unload them when not needed.
Overall, even though I have extensive exterior environments I still try to follow certain guidelines that people use for interiors. If i can hide environments in the distance by a jog in the landscape or behind rocks, or over a hill it helps to turn it off unless needed (streaming). I make my buildings more extensive than I originally did to allow for areas to steam in and out.
Part of all this is working smart, and pre-planning. If you can build LOD models while you make modular models, and reduce textures to allow for lower resolution Material culling while you work it will tend to build up less, you may even be surprised that switching to a 512x512 texture map shows no noticeable loss of imagery in some areas, instead of using higher resolution, but it depends on the textures you’re using of course. My textures tend to be repeatable textures that allow me to use them many times over, so that they show less flaws when reduced.