You guys know, many of downloaders dislike FLV more than mp4.  >:D
So if you would like to upload FLV torrent and wish it would get long life, you'd better to consider that.
And I'll tell you, it's not difficult at all. 

software:

• FLV Extract

• AVI-Mux GUI

setup:

• 1-1. Extract FLV_Extract.zip

• 1-2. You can delete 'Source' folder, if you aren't interested in source code.

• 2-1. Extract My MP4Box GUI v0.5.6.0.7z

• 2-2. Install with Setup.exe

• 3-1. Extract AVI-Mux_GUI-1.17.8.3

extract and mux:

• 1. Startup FLVExtract.exe

• 2. Drag and drop flv to FLVExtract's window.
    Then you'll got separated video and audio files.

• 3. If extracted files are avi and mp3, startup AVIMux_GUI.

• 4. Drag and drop both files to upper list-window.

• 5. Click "generate data source from files" button.

• 6. Click start button.

• 7. Select folder to save and specify file name.

• 8. Done!

note:
if extracted files are h264/AVC raw file and aac, you need other muxing tool.
I recommend Yamb (GUI for mp4box).