This performance gap can be explained very easily: A8 IS very, VERY outdated - that means there are only so many ways you can torture your hardware with it. If you build a scene that runs well in A8, it probably is not very complex by modern standards.
Now, you port this not very complex scene to UE4, an industrial-grade, state of the art engine, and suddenly it performs WORSE. How is this possible?
Well, the short answer is, UE4 is very very potent, offers A LOT of features, very modern technology, and these features simply come with a performance cost, even if you do not make full use of them. And if your PC is a bit older, it will have problems. UE4 is not designed to run on old machines out of the box, you can make it perform well on older hardware but you'll have to enforce some optimisations that it may not apply per default.

However, a very complex scene, using a lot of effects and features that UE4 offers you, and that runs well on a modern machine, will run very slow, if at all, when ported to A8, EVEN on the modern machine.

Long story short, UE4 has higher baseline hardware requirements than A8, yes. If your PC doesn't meet them, you will have poor performance no matter how complex your scene, and you may get better performance in A8 as long as your scene is not too complex.
To make use of UE4s potential you will have to upgrade your machine.