@Anonymous User
On most NowTV playback devices including the NowTV inbuilt App on the Samsung TV's it's 720p (see link below for maximum streaming quality across different devices).
https://help.nowtv.com/article/what-is-highest-streaming-quality-for-my-device
The quality may differ between different brands and models of TV's depending on the televisions up scaling electronic processing (My Panasonic UHD TV does an excellent job with upscaling a HD 720p signal to the television native display of 3840 by 2160) 
If you are using say a nowtv black box against your Samsung NowTV app then because the TV is using the same inbuilt up scaling electronics on your Samsung UHD TV, then in theory you shouldn't see much or no difference to your naked eye.
