mp4 the audio or video format? Some audio files have the mp4 file extension. Apple uses .m4a for mp4 audio format so it's easy to know the difference.

This codec pack looks very good K-Lite Codec Pack 4.7.0 FULL / STANDARD/ BASIC : K-Lite Codec Pack is a collection of codecs and related tools (includes video & audio codecs.)

Or this one FFDShow MPEG-4 Video Decoder 2009-03-01

However, not good to mix brands of codecs. Best to keep to one brand. Otherwise clashes can occur. That happened to me, one codec pack refused to run at all, Windows XP system said it was not even on my system. lol