This page covers the basics of using models in UE4. In general, you can do anything with them that you can with any static mesh.
Once you've imported the model as an asset you can place it in your scenes like you would any other static mesh. The most common methods are:
Place them manually. Simply drag the asset and drop it in the scene to place a model. This gives you the most flexibility in how it is lit. Mark them as “static” (you'll need a good lightmap <link>) to use them with Lightmass or “moveable” to have them be dynamically lit and shadowed.
Paint them as foliage. Drag the asset to the foliage painting area and paint the models like any other static mesh.
Note: Painted geometry is instanced and renders very quickly; however, it cannot be lightmapped. Keep this in mind as you populate your scenes.
Add wind to the scene by add a wind direction source object (Select “Window→class viewer”, search for “WindDirectionalSource”, and drag it into the scene). The “Speed” property defaults to 1.0 which is considered maximum wind for SpeedTree models. Lower this value to something like 0.1 to start with calm wind.
LOD transitions are based on screen space area in UE4. These values are edited by editing the static mesh asset associated with the SpeedTree model. Default values are chosen upon import that work for typical situations.