There is quite simple explanation for that..
Item properties shows active decoder for mp4 container.
It could be either VLC library, FFMPEG library or system (DirectX, Quicktime) decoder.
Preferred one can be set at Preferences -> /Video/Rex/Misc as video decoding engine.
It is also used for MP4 with only audio track. So for instance AAC files from Apple (iTunes) are also possible to directly open via these decoders, it is only necessary to change file extension from .m4a to .mp4.
To to previous quesion.. regarding necessary installation of VLC. It is not necessary, but often easiest solution for working decoder. But you can of course use other ones.. So Quicktime installation with or getting FFMPEG libraries.
FFMPEG needs to be downloaded separately (latest working 1.2 version is there)
http://ffmpeg.zeranoe.com/builds/win...in64-shared.7z
or
http://ffmpeg.zeranoe.com/builds/win...in32-shared.7z
for 32bit platform.
Then just extract all .dll's from bin directory in archive to Reaper's plugin folder
eg. C:\Program Files\Reaper\Plugins or %APPDATA%\Roaming\REAPER\UserPlugins\
and similarly for 32bit versions.
Which decoder option is best is up to test with particular format or personal preference..
Michal