FFmpeg, encode video for 720p @ 4mbps playback
The ffprobe command below will spit out the bitrate:

ffprobe in.mp4 -select_streams v -show_entries stream=bit_rate -of compact=p=0:nk=1


e.g.

4118620


If the result is greater than 4000000 then you can add -b:v 4000k -bufsize 4000k -maxrate:v 4000k. Or you can add it anyway. Set -c:v to copy if the stream is under 4000000 else to libx264. The three values will be ignored just like -b:a:0 448k in your command is.