What are you using to play the movies on your TV?
Any relatively recent device (Roku/AppleTV/FireTV/Shield) should be able to direct play your movies so you don't have to worry about transcoding.
Then your i3 will be plenty.
You'll also want to make sure your NAS is connected to your network via ethernet, not over wifi. Trying to stream 4k when both your NAS and playing device are connected over wifi will be a buffering mess, regardless of how powerful your processor is.