I saw a video that the Tesla is in Forza 4, but I have no Idea how to unlock it. Does anyone know what to do to unlock the Tesla?
It appears to be part of a pre-order DLC - the Launch Bonus Car Pack. See this blog post on the Forza Garage
Cars included in the bonus pack:
If you pre-ordered, (or perhaps bought one of the first run of copies at retail? There's some ambiguity here) then you should have gotten a code in the box for this. There does not appear to currently be any other way to get these cars in the game.