user12145: I have to issue a series of commands, all in the form as shown below:

ffmpeg -i 1.mp4 -s 640x480 -c:a copy 2.mp4
  ffmpeg -i 2.mp4 -s 640x480 -c:a copy 2_2.mp4
  ffmpeg -i 3.mp4 -s 640x480 -c:a copy 3_2.mp4
  ffmpeg -i 4.mp4 -s 640x480 -c:a copy 4_2.mp4  

Please show me how can I issue all commands automatically or do it with one command.

user1760: use a loop
If you're a Linux or OS X user then you can use a Bash for loop:
mkdir outputdir
for f in *.mp4; do ffmpeg -i "$f" -c:v libx264 -vf scale=640:-2 -c:a copy "outputdir/${f%.*}.mp4"; done

Maybe a Windows user can add a similar example that works for that OS.
or simply cram it all in one command
ffmpeg -i 1.mp4 -i 2.mp4 -i 3.mp4 -i 4.mp4 \
-map 0 -s 640x480 -c:a copy 1_2.mp4 \
-map 1 -s 640x480 -c:a copy 2_2.mp4 \
-map 2 -s 640x480 -c:a copy 3_2.mp4 \
-map 3 -s 640x480 -c:a copy 4_2.mp4

You have to use -map otherwise the default stream selection will choose the same input for each output.

12145: <p>I have to issue a series of commands, all in the form as shown below:</p>

<blockquote>
  <p>ffmpeg -i 1.mp4 -s 640x480 -c:a copy 2.mp4<br>
  ffmpeg -i 2.mp4 -s 640x480 -c:a copy 2_2.mp4<br>
  ffmpeg -i 3.mp4 -s 640x480 -c:a copy 3_2.mp4<br>
  ffmpeg -i 4.mp4 -s 640x480 -c:a copy 4_2.mp4  </p>
</blockquote>

<p>Please show me how can I issue all commands automatically or do it with one command.</p>
 1760: <h1>use a loop</h1>

<p>If you're a Linux or OS X user then you can use a Bash <em>for loop</em>:</p>

<pre><code>mkdir outputdir
for f in *.mp4; do ffmpeg -i "$f" -c:v libx264 -vf scale=640:-2 -c:a copy "outputdir/${f%.*}.mp4"; done
</code></pre>

<p>Maybe a Windows user can add a similar example that works for that OS.</p>

<h1>or simply cram it all in one command</h1>

<pre><code>ffmpeg -i 1.mp4 -i 2.mp4 -i 3.mp4 -i 4.mp4 \
-map 0 -s 640x480 -c:a copy 1_2.mp4 \
-map 1 -s 640x480 -c:a copy 2_2.mp4 \
-map 2 -s 640x480 -c:a copy 3_2.mp4 \
-map 3 -s 640x480 -c:a copy 4_2.mp4
</code></pre>

<p>You have to use <a href="http://ffmpeg.org/ffmpeg.html#Advanced-options" rel="nofollow"><code>-map</code></a> otherwise the default <a href="http://ffmpeg.org/ffmpeg.html#Stream-selection" rel="nofollow">stream selection</a> will choose the same input for each output.</p>
