[cathkun]

FLV Editor is the most powerful and easy-to-use program for editing FLV
and F4V files. You can use FLV Editor as a FLV Combiner, FLV Joiner, FLV Merger that means join several different .flv files together into one file for easy
-watching. FLV Editor generates HTML/SWF (with various player skins,
special effects, and preloaders) for uploading to website and blog.
Key Features:
1. Combine FLV F4V files encoded with H.264, H.263, Flash 8, or Flash 8 with alpha.
2. Extract FLV audio or video. Or replace the original FLV audio.
3. Trim FLV/F4V/MP3 to get the desired segment.
4. Generate SWF and HTML files for uploading website and blog.
5. Save FLV/F4V files as .mft project for further editing.
Guide on how to combine FLV F4V files into one file?
Step 1: Download and install FLV Editor on your PC, then you’ll see the friendly user-interface as below:



Step 2: Import your FLV, F4V, or MP3 files that wanted to be edited to FLV editor.



Step 3: Drag your original FLV files to the Timeline and arrange the order by dragging each edge of .flv file.



Step 4: Export your source FLV files to new FLV or SWF file by click Export FLV or Export SWF icon on the top menu.

[soft28]

FLV Editor not only can edit flv video/movie with a few clicks, such as trim, crop, watermark, effet and merge multiple flv files into one, but also can convert the flv files to other video formats with superfast speed and high quality.

[Dathelee]

You may try Free FLV Editor which has many great functions. The features in the official site are below: FLV Editor Lite is a powerful freeware to edit and personalize your Flash videos. It is a useful FLV/ F4V movie editor which provides the most flexible FLV editing solutions. Free download and free to edit FLV/ F4V/ MP3 files.

Maybe you just downloaded some pieces of a movie and want to enjoy them without break. Use FLV Editor Lite! Just import the FLV files in order and merge. They will be joined into a single one perfectly with the original quality. With the help of FLV to Video Converter Pro 2, you can even convert the file into other format for playback on DVD player or portable devices like iPod and iPhone.

Maybe you are making an MTV, and want to replace the background music.Do it with Moyea FLV Editor Lite! Just download your favorite video clips from the video-sharing sites with the free youtube flv downloader Import them into this program, select the desired parts by trimming and merging, and then replace the audio with your favorite one. An attractive MTV will come into your eyes in the real-time preview pane. If generating SWF file, you can even add stunning player skin, special effect, and preloader. Uploading the generated FLV, SWF and HTML files, you can upload online to your website or blog to show that you are an expert on MTV making.

Maybe you are crazy about online videos and would like to edit FLV/ F4V files from YouTube or other sites.It is so easy with FLV Editor Lite. Download and import the FLV movie, and you can do whatever you want, split the file into several pieces, rearrange, insert other clips, merge FLV, replace audio, and so on.

Maybe you are a Flash enthusiast and want to add some unbelievable effects to your Flash video. Get the help of FLV Editor Lite! With the cue point function, you can add any events and realize whatever Flash effects assigned by ActionScript.

There is step by step tutorial:
Flash-Video-Soft Blog Archive How to merge FLV files free?

Also, you can use adobe premiere which may be difficult for us: