In UE4:
When you import a rigged model (mesh parents armature) to UE4, it will be converted to a Skeletal mesh, a skeleton and a physics asset
An animated FBX will be converted to the 3 files mentioned above and anim sequence.
(You simply export the entire animation from blender into UE4, now UE4 will have an import dialog where you can specify the start/end range)
Blender Animation keys might be off
So you do not need to reimport the skeleton every time you want to create a new animation. You specify what skeleton you want to use, and UE4 will retarget the imported FBX animation onto the skeleton
In Blender:
Only select the object you want to export
re-target the anim and remove the duplicates later.
set the scale to centimeter
You can always reset the axis in the export tab, UE4 uses the classic Pitch/Roll/Yaw, Unity uses CG way(Y: vertical, Z:depth, X:horizantal)