The things you have installed on your webserver have nothing to do with whether or not the client can play the video served by your server, that depends on what you implement on the client side.
Besides that the FLV container format is becoming deprecated with the increased support of HTML5 and I wouldn’t recommend using FLV in any new web project that doesn’t need high compatibility to absolutely every older browser.
MP4 is supported in all major browsers except for Opera, it doesn’t need any plugins or special players on your website, you can just use the HTML5 <video> tag.
Its extremely easy to implement and is very well supported.
If you have questions about how to setup things in wordpress you should better ask that on Stackoverflow instead.