MP4 format is well supported by many portable video players like iPod, iPhone, Zune, PSP, Blackberry, etc. while HD MKV(Matroska Video) is commonly used to store video, audio and subtitle data on computers that can't be well supported by most media players. So, if you want to play your favorite MKV movies on your PMP (portable media player), it's necessary to convert MKV to MP4. And here is an easy way to convert MKV video to MP4. You could even add subtitles to MP4 video with ease.
To change MKV format to MP4, you need a Video Converter. Here Video Converter Ultimate is highly recommended to help your convert your MKV video files to MP4 (MPEG-4) and other formats like AVI, WMV, FLV, 3GP, ASF, MKV, M2TS, MTS, etc. with plug-in subtitles in one click. If you are using a Mac, you can turn to Video Converter for Mac to change MKV to MP4 with subtitles on Mac (Lion). Now download the software and follow the steps below to complete the conversion quickly and easily.
Install and run the Video Converter Ultimate, then click the "File" button to load the?MKV Movie files you want to convert from your computer to the program. Or you can drag and drop video files directly to the app. It well supports batch conversion, so you can add multiple files. After adding the files, you can preview the source video in the preview panel on the right of the main interface.

Click "Profile" drop-down list on the main interface to choose "MPEG-4 Movie(.mp4)" under the category of "Common Video" as output format. You can click"browse" button to set a destination folder for these target files.

Click "Edit" button, switch to "Plug-in Subtitle" tab. Just select "enable the plug-in subtitle" checkbox, and then you can add plug-in subtitles to MKV files. The supported subtitle files are .srt, .ass, .ssa files.
Just click the big "Convert" button to begin converting MKV video to MP4 with ease. After the conversion is finished, you can click "Open Output" to access to the output files directly. After that, transfer the videos to your portable devices and enjoy them!