Better plan on getting the 128GB 6s / 6s Plus.
Based on the sample 4K video they put on the 6s page, it's 381.6MB / min.
That means you could (not talking into account the OS or other apps—this is just rough math) hold less than 42 minutes of 4k video on the 16GB model, 172 minutes on the 64GB, and 343 minutes on the 128GB.
4K video sample from here:
http://www.apple.com/iphone-6s/cameras/
Based on the sample 4K video they put on the 6s page, it's 381.6MB / min.
That means you could (not talking into account the OS or other apps—this is just rough math) hold less than 42 minutes of 4k video on the 16GB model, 172 minutes on the 64GB, and 343 minutes on the 128GB.
4K video sample from here:
http://www.apple.com/iphone-6s/cameras/