title: ffmpeg combine video streams into 1x4 collage
body: <p>Can anyone please help understand how to make correct <code>-filter_complex</code>
  expression in <code>ffmpeg</code> to join 4 RTSP streams in a row collage 1x4? 
  The complexity is that there is ONE input with 4 video streams inside.
  <code>ffprobe</code> output:</p>
  
  <pre><code>Input #0, rtsp, from 'rtsp://MyStream':
    Metadata:
      title           : h264.sdp
    Duration: N/A, start: 0.024944, bitrate: N/A
  Stream #0:0: Video: h264 (Baseline), yuv420p, 640x480, 27.92 tbr, 90k tbn, 180k tbc
  Stream #0:1: Video: h264 (Baseline), yuv420p, 640x480, 27.92 tbr, 90k tbn, 180k tbc
  Stream #0:2: Video: h264 (Baseline), yuv420p, 640x480, 27.92 tbr, 90k tbn, 180k tbc
  Stream #0:3: Video: h264 (Baseline), yuv420p, 640x480, 27.92 tbr, 90k tbn, 180k tbc
  </code></pre>