Hey @Aaknar! Welcome to the forums!
So a little information first:
It is generally not safe to migrate files backwards in version (5.4 to 5.3)
It can be done but it is a headache. You would have to export the character to Blender or Maya, then export it from there as .fbx and import it to the 5.3 version, and even then there’s still likelihood of a few small hiccups.
You cannot Merge two projects. A .uproject file is not mergeable with another .uproject. That’s the program, that’d be like trying to merge two .exe files.
What I would suggest is finding the rendering scene map file in 5.3, right click it and use “Asset Actions → Migrate.” Then find the 5.4 project that has your character in it, and migrate the scene there.
Hope that helps! 