MP4 files represent one of the current multimedia standards, mostly for video files. Similarly to what MP3 achieved in the past, MP4 improves greatly video performance, getting rid of some limitations of previous formats. Then, how can you use those files?
The easiest way to play MP4 files is to use some device that can decode it. This is the case for modern portable players, that will also work with other standard formats as WMV (Windows Media Video, from Microsoft) or ASF. Also some home-entertainment systems can already decode and play MP4 files on your standard TV set. It is important to note that MP4 does not define a single file format, but it is a container, which defines the basic parameters and conditions for the embedded media. In the same manner that DivX players reproduce AVI files compressed with the DivX codec, MP4 players will decode some different compression methods, such as MPEG4, DivX, XviD, or some specific ones, such as h264 format.
The alternative is to watch MP4 files on your computer or on your TV, from the computer through HDMI, for example.
There are many different players in the market. Even the multimedia players included with the operating systems such as Windows Media Player can reproduce this kind of files. Only you may need to upgrade the software, by adding some third-party codec that can translate MP4 containers into a video stream on your screen. In most cases, the specific media player will search the Internet for the needed update, and it can usually be at no cost at all. In general, you should avoid paying for this kind of upgrades, unless those are to be used commercially.
Alternatively, there are some applications in the market such as VLC media player, which are free and already include compatibility to most of the current media formats, that is, both audio and video. User interfaces tend to be simple but only because they hide unused commands. Typically, powerful utilities like sound mixer and multi-channel audio support are included, but not active upon start.
The main advantage of programs such as VLC is that they will use just a few resources from your computer like the micro controller time and RAM memory, so that even HD video can be fairly reproduced on modest computers.