download and install the free converter tool Mpeg Streamclip …
Step #1
export your Project from within iMovie (cmd-E) in any size you wish
Step #2
open resulting .mov with Mpeg Streamclip
Step #3
go straight to 'Save as' and select 'mp4' from drop-down menu
This process is very fast, because it doesn't convert the video, but re-wraps it.
The new .mp4 is compatible with any Windows flavor, XBox, PS3, Linux, …, etc etc ......
and, no, just changing the suffix .mov >> .mp4 doesn't do the trick.

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