The first time you export to MP4 using the default settings, FlashBack will download the openh264 codec. In most cases, it should just take a couple of seconds to complete the download. No further downloads will be required in the future.
Generally speaking, this codec will give better results than the MS Media Foundation codec that is included in Windows but, for licensing reasons, it must be downloaded separately - we are not permitted to include it within the initital Flashback installation.