Apple's new iPhone-----iPhone 4S now fully support HD video playing and up to 1080p. But we can know from its official website that no matter 1080p or 720p support, its formats should be limited to M4V, MP4, MOV formats or special Motion AVI format.
If you have many MKV movies download on-line on store from splendid TV shows, they are HD MKV files in 720p or 1080p. How to watch them on iPhone 4S Retina Display? You will use the little tool named Mac MKV to iPhone 4S Converter which helps you convert HD MKV to iPhone 4S supported MP4, MOV, H.264 and keep the original MKV HD 720p or 1080p Quality.
Import MKV videos to MKV to iPhone4S Converter from your PC HD drive.
To set iPhone 4S supported format as output.
Go to HD option------Set H.264 1080p/720p
Go to iPhone option ----Set MP4 480p
Go to iMovie option------Set MOV
Click "Start" button to begin MKV to iPhone 4S Conversion.