>>29079197
4MB = 4* 1024* 1024 * 8 bits
divide by number of seconds
that's your ceiling bitrate in bits/second
divide by 1000 for kbps
the right bitrate is based on your codec (vp9 would be better than avc), quality of source (low quality/pixilated/compression-artifacts means not much point trying to keep bitrate excessively high), and how compressible it is (slow action is easy to compress, fast action is harder), resolution and length