user5347: If I have an existing MP4/H264 video file and I need to trim the start and end,
Then save it as an MP4 again,
Are there any recommendations on export settings?

user1871: 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.

5347: <p>If I have an existing MP4/H264 video file and I need to trim the start and end,</p>

<p>Then save it as an MP4 again,</p>

<p>Are there any recommendations on export settings?</p>
 1871: <p>You can use <a href="http://ffmpeg.org/download.html" rel="nofollow">ffmpeg</a>, a free command-line tool, to do this</p>

<h3>Without recompression</h3>

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

<p>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.</p>

<h3>With recompression</h3>

<pre><code>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
</code></pre>

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