Play 1 first. It's kind of clunky today, but it's still a great game. Then play 2 for a while just to appreciate 3 more. 3 is the best one, and it's also a good place to start if you just want to go straight to the cuhrayzee. 4 has the best combat system, but has some other problems. Still, it's a worthy follow-up to DMC3.
Other than 2 they're all good in their own ways. Just don't expect them to be ANYTHING like DmC.
Not changing this sig until Little Big Adventure 3 is announced. In other words, never.