How to get fragment of mp4 file? How to prope rly concatenate mp4 files?

Hi.
 
*  Given some mp4 file, I need to retrive fragment from it. Say I’m interested  of a fragment on k second of a video file, I need to retrieve video fragment  from k-2 second to k+2 second, namely few seconds before interested event, few second after interested event.  What is the best approach (pipeline)  to do that?
*  What is the correct way to concatenate mp4 files, is it simple stream concatenation — append one file to another or there could be corner cases?
     
Thanks in advance.