Season 1 - It is not real 4K in any material way due to having a final master at 2K. Studios have upscaled the 2K image to 4K and retouched the content for your 4K UHD TV. You should see some visual improvement over a standard 1080p Blu-ray.
Seasons 2+ - This is the real deal. Everything was digitally shot in 4K or the original 35/70mm film negative was scanned in 4K and all the mastering/editing was done in 4K. Typically, if the film has VFX it was rendered in 2K. Depending on the caliber of CG, the final picture quality might be imperceptible to Real 4K.