Putting a Lower bitrate of my video stream in my current mp4 container
Answering the question as asked,

ffmpeg -i input.mp4 -map 0:v -map 0:a -map 0:v \
-c:v:0 copy -c:v:1 libx264 -b:v:1 &lt;lower-bitrate&gt; -c:a copy -movflags +faststart out.mp4


The lower-bitrate version of the video is the third stream. Players typically will play the original video which is the first stream in the output.