Not an expert but I grabbed a rigged, weighted character from Mixamo.com and just using the FBX import I applied the new weighted mesh with ease to the third person example UE4 skeleton.
In the content browser I clicked import and selected the FBX. In the import dialogue I selected the skeleton my character/animations were set up on (basic UE4 Tim Plate / Manny Quinn skeleton). On a basic level the job was done on import. Some bones were different but the difference was negligible and didn’t effect the animations I was using - your mileage may vary. Jumped into the character class and set the new mesh in the mesh component and hey presto.
In the character BP under the skeleton tab there is also a re-target manager to help with this stuff. Individual animations can also be re-targeted by right clicking the asset in the content browser and selecting ‘retarget anim assets and duplicate’ which I have found useful.
Lot of good stuff in Unreal->Learn->Documentation