In addition to Asoeli's answer you will have one of two possible sets of export options available, depending on the codec you are using.
If you are using the Intel (Default) codecs then the options for MP4 export are as shown below in the red square - you will not have the top 2 in the list - these are custom exports that I have created - you select one of the MP4 options.
If you are using the Main Concept codec then the options are as shown below - you select one of the FlashPlayer, HDTV or PC options for mp4 - note the Flashplayer options will give smaller files but at lower quality