I don't own an Apple 4K device, so i won't be much help if you have already done the basic stuff like, go into the NowTV movie by pressing the pause or info button to bring up the on screen menu and see if the bubble speech icon is highlighted or not.
See if there is anything in the Apple TV box settings under Captions that needs changing.
Maybe try a power down reboot of the box or a fresh NowTV App download if you haven't done these above items already.
Hopefully an Apple TV user will come to your rescue.
I started playing around with the Apple TV subtitle settings and found it was down to Apple TV. They were however, meant to be turned off as that was the setting in Accessibility. I then found in Video and Audio settings -> Audio -> Subtitle Language - this was set to Auto. I changed to OFF and subtitles have now disappeared. I think it’s an Apple software glitch but this workaround does the job.
Thanks for your help and suggestions! 🙂