如何使用FFmpeg或mencoder或其他方式按大小拆分flv文件？

I need to split an flv file into chunks of the known size on linux server. For example my original file is 9Mb and the chunk size is 4Mb. So I should get 3 parts - 4Mb, 4Mb and 1 Mb.I need to split an flv file into chunks of the