Best way to trim and save an existing mp4
You can use ffmpeg, a free command-line tool, to do this

Without recompression

ffmpeg -ss 02:45 -t 03:05 -i orig.mp4 -c copy -map 0 -avoid_negative_ts make_zero cut.mp4


This will copy a 3m 5s portion of the original MP4, starting at 2m 45s. Since the codec of the video stream in a MP4 usually uses temporal compression, the cutpoints may not be exact and some frames before and after may be included in the trimmed file. Since there's no recompression, quality is identical to the original.

With recompression

ffmpeg -ss 02:45 -t 03:05 -i orig.mp4 -c:v libx264 -crf 20 -c:a aac -b:a 128k -map 0 cut.mp4


This will precisely cut the specified portion but will recompress the content. The crf value controls the video quality - smaller value produces better quality but a larger file. The b:a sets the bitrate of the audio output.