Convert MP4 to FLV on ubuntu

In Ubuntu, start Synaptic Package Manager, search the following two packages, and mark for installation.

ffmpeg
This package contains the ffplay multimedia player, the ffserver streaming
server and the ffmpeg audio and video encoder. They support most existing
file formats (AVI, MPEG, OGG, Matroska, ASF...) and encoding formats (MPEG,
DivX, MPEG4, AC3, DV...).

avidemux
Avidemux is a free video editor designed for simple cutting, filtering and
encoding tasks. It supports many file types, including AVI, DVD compatible
MPEG files, MP4 and ASF, using a variety of codecs. Tasks can be automated
using projects, job queue and powerful scripting capabilities.