Oh sorry I assumed you were talking about a 3D environment. If you’re making a side scroller, I’m guessing it will either be flat 2D in which case you won’t need a landscape object not sure how they make long landscapes in 2D, possibly using tiled landscape images? Or possibly 2.5D, in this case you might as you say use a long thin landscape for the ground which you could sculpt to make the landscape you want using the UE4 landscaping tool, then place your asset models along the map to create your level. In which case you will most likely use individual mesh models for the various map props (trees, bushes, fences, buildings, etc) how modular you make each prop, for example buildings, would be down to preference. Then how you transition from an urban to rural area on your map will be down to artistic style and how you transition from using the set of urban models to using the rural ones. As or making the models Maya would probably be preferable but otherwise Blender or 3Ds Max.