Yep! What Fighter said. Basically in the Levels window, you can highlight all of your levels, then click the Details option. The bottom half of the details panel has options for generating LODs of your Landscape Actors. You can customize how/when they’re loaded in and out, and to what level of detail they are. This is free and built into UE4, no Simplygon required. However, this does NOT generate LODs for the actors/meshes within each of your landscape tiles (trees, bushes, etc). That’s where simplygon would come in handy I believe, but it’s fairly easy to create your own LOD’s for your own assets! Good luck.
Also, if I’m not mistaken, this feature is only available in 4.8 +.