Do artist maintain ownership rights to their creations?
That’s a really broad question, quite impossible to give an answer with no info whatsoever.
But yes, of course, generally, if you make a drawing, animation, 3D model or anything, others can’t use it without a license which permits them to do so.
It’s a bit more complicated if you’re working for a company or if the software you’re working with specifically states that your rights will be limited.
Generally, the same rights apply to UE4 artist creations than to any other creation, so you can just look up ownership right for intellectual property, and that’ll most likely apply to you too.