UE4 is dependent on both, but a better GPU will be a bigger upgrade performance wise for most of the games you create.
The CPU runs the Physics calculations, will affect lighting build times, and compile times, and the GPU will do all of the the graphics calculations such as drawing the materials and meshes in your scene.
In general a better GPU will give you a higher FPS increase, but you don’t want to skimp on the CPU either. Hope that helps! 
EDIT: I see ENiKS says “Stay away from ASUS products if you ask me”.
I am completely the opposite, ASUS is in my opinion the best out there when it comes to hardware. I have purchased many different parts over the years from ASUS and have never had any issues whatsoever. I can’t speak to how good their laptops are since I have never owned one, but if I was going to buy a laptop they would be my first choice when researching various brands.