Key concepts of Unreal Engine 4 and Unity :

Game logic

Unreal Engine 4

Game logic is written using C++ or Blueprint Editor. C++ class and blueprint manipulates the objects. Object has only one class and blueprint is connected to this class.

Unity

Game logic is written using Mono environment. Scripts manipulate the objects. An object has a lot of scripts or does not have any.

Scene

Unreal Engine 4

Z axis up.

X forwards, backwards
Y left, right
Z up, down

Unity

Y axis up.

X left, right
Y up, down
Z forwards, backwards

Scene Objects

Unreal Engine 4

Base scene object - Actor

The base object that can be placed into the scene is know as actor. One Actor can has a lot of components of same type. Most of components support parent-child relation.

Unity

Base scene object - GameObject

GameObject is container for all other Components. Components are then added to the GameObject. GameObject has not same type of components (except scripts). Components do not support parent-child relation.

Base Datatypes

Hope it will help you.
Thankyou !