While Simplygon is not available for UE4, InstaLOD is! InstaLOD, an automatic LOD generation and mesh optimization solution is now available as a plugin for UE4 in early access. It ships with source code to allow you to use it with the launcher version of UE4 as well as your custom UE4 forks.
InstaLOD enables you to built LOD models directly in UE4 with full support for UE4’s HLOD capabilities introduced in 4.11. You can easily reduce your draw calls by merging multiple meshes into a single draw call by combining both polygon data and materials into a material atlas.
More information can be found on our website: http://www.instaLOD.io
And on our thread on the UE4 forums: https://forums.unrealengine.com/showthread.php?98648-PLUG-IN-Automatic-LOD-Generation-for-Static-Meshes
TL;DR: InstaLOD is an awesome automatic mesh reduction tool now available for UE4 www.instaLOD.io
Disclaimer: I am one of the developers of InstaLOD 