Key moment:
Download the Anim pack on Epic (its mesh is old UE4 mannquin, while the reference character mesh used for retargeting is UE 5 mannquin)
Create Rig file and IK rig for targeted character:
Use the reference UE 5 mannquin (open a default project) to check the pelvis (treat it as a core of the mesh, R click →set Retarget Root)
Then for root, spine, clavicl, etc. R click → New Retarget Chain from Selected Bones
Final newly added IT Retargeting chain:
Create IK rig for old Mannequin:
Repeat the same step on old UE 4 mannequin mesh:
Use IK retargeter to Retarget the anim:
i.e. Use old (UE4) mannequin’s IK retarget as a source for the anim