Thus all visitors who come to your website will be able to watch it. It will play on any portable device and in any browser. It is a universal format supported by literally all modern video utilities and gadgets. In case you plan to edit your video in a professional video editing tool and\or upload the result to your webpage, your choice is MP4. MP4 question is not clear for you, think about your goal. Click the weight button right above the "Convert" one and set the necessary output size.
A full version can be installed on PCs not connected to the Internet. The online version is your choice if your connection is fast.
The installation file is available in two versions. Download the free software from Freemake website. The freeware is ad-free, easy-to-use and works like a charm. Freemake FLV to MP4 Converter is right what you need.
To encode FLV videos to MP4 format, you need a stable and reliable tool.