Record video in QHD then reduce to FHD is the same thing of record in FHD I have a Galaxy S7 Flat, and I use Google Photos Unlimited backup free (which means at max/reduce videos to Full HD).

So I was wondering if I record in Quad HD (4K) has the same quality of recording​ in Full HD since the beginning.
Quad HD recording is superior to full HD as you can see from table



Source

But if Google photos is reducing it Full HD, then the quality will be superior to that of a video recorded in Full HD and uploaded based on Google Product forums  


  Videos higher than 1080p will be resized to high-definition 1080p. A video with 1080p or less will look close to the original.


(Emphasis Added)

In other words, if the quality of recording is less than or equal to 1080p, it will suffer  compression losses, not if it is higher