(Note for Studio users: OBS Studio can remux FLVs to MP4 as a built-in feature. Just go to File > Remux recordings, select your FLV, and click Remux to convert them quickly to MP4.)
A lot of streamers like to save a local copy of the video they stream out in order to edit it later, upload it to YouTube, or just keep an archive. Usually, this means saving the video as an MP4 in your Broadcast settings. However, we know that OBS is beta software, and streaming is a complicated, resource-intensive process, and sometimes things crash. If you're recording an MP4, and OBS crashes before the MP4 can finish saving, it will be corrupted. FLVs, however, don't have this problem, because the container is designed to be started and stopped at any time. So if you are recording an FLV and OBS crashes, the FLV should not get corrupted.
Furthermore, users of OBS Studio might notice that (for the time being), FLV output is your only recording option. So you might need a way to convert FLVs to MP4 format.
FLVs and MP4s are actually very similar. They both are simply containers for an h.264/AVC video track and a AAC audio track. So the only difference is the container itself. Thus, you can very quickly and easily switch containers without needing to re-encode the video or audio!
Step 1: Download FFmpeg. You can get the latest Windows version from here or the latest Mac version from here. Extract it somewhere that you will remember. If you're on a Mac, I recommend putting ffmpeg into the folder where the video is that you want to convert. For Linux, install it from your favorite package manager or install compile it from source.
Step 1a (Windows, Optional): Add the /bin folder where you extracted FFmpeg into your PATH environment variable to make it easy to access (Control Panel > System > Advanced System Settings > Advanced Tab > Environment Variables > System Variables > PATH > Edit, put a semicolon at the end and type the new path)
Step 2: Open a command prompt/terminal and navigate to where your video is stored.
Step 3: Run the following command (If ffmpeg is not in your path, you will have to type out its full path):
Replace "input.flv" with the name of your source FLV file.
Replace "output.mp4" with what you want the name of your resulting MP4 should be.
The "-c copy" means it will just copy the audio and video tracks without re-encoding them.
The "-copyts" flag means it will copy timestamps, which should help with syncing audio and video.
Step 4: There is no step 4. You're done! That was fast.