Blender is a free and open source tool you can use to model, texture, and animate 3D objects.
Blender supports several export formats, but the two most relevant to UE4 are FBX and glTF.
- Powerful and flexible, but hard to get right.
- You must answer questions about coordinate systems and units.
- Lot of options, but you can save a Preset for your export options.
- Blender FBX export code and UE4 FBX import code is difficult to understand and buggy.
- T68575 FBX Known issues & TODO's.
See Blender FBX Pipeline for more details.
- Standard defines coordinate system and unit scale
- Exporter handles converting to that standardized coordinate system, simplifies importer and user options.
- Models, textures, materials.
- Format can handle static models and animated characters, but UE4 side is still working on the animation part. For now, use FBX for now for animated characters.
- Does not support collision or sockets. 
- Mr Mannequins Tools - Create UE4 mannequin compatible animations in Blender
- Blender User Manual
- Blender Python API
- Blender YouTube channel
- Blender Conference
- Blender + Unreal asset pipeline foundations - Mike Erwin - Blender Conf 2019
- Blender to Unreal Tools Part 1, Part 2, Part 3
- Blender to Unreal Engine with Matt Workman
- Blender to UE4 (2017)