You don't need to play any others, as they're set hundreds of years apart and don't really have a lot to do with each other. There's references to events in the previous games, but that's about it.
The first four all take place in the span of 40 years or so as someone pointed out. All the events are intertwined and part of a prophecy written out on The Elder Scrolls. The four events that happened are what lead up to and cause what is happening now 200 years later in the latest game. It's a bit complicated to understand but it isn't necessary to play the others to play and enjoy this one.
The main story of an Elder Scrolls game isn't really even that important and makes up only about 10% of the game...if that. If you care about story this is a wonderful Wiki available for viewing or you can read any of the HUNDREDS of in game books.
They've also made many improvements over the old games so I would suggest playing them first for at least just the story if you ever plan on playing them. That way you aren't pissed and disappointed at how broken and badly built the leveling system was back then. Now it's pretty much perfect.
All in all though if you do plan on playing, which I highly suggest you do, make sure you have a lot of free time dedicated to it and don't expect to do much else other than play this game.
