So classic that it's practically coin-operated.
In Contra 4 you fight against an army of evil alien invaders, who kind of look like people. Of course you are the last hope of mankind to stop the invasion. Story is a classic Contra style: simple and straight forward, but that's the way it has always been.
Graphics resemble those old SNES games so in terms of graphical presentation Contra 4 is not very good. Still you know what's going on and the use of bright colors is mostly good. Sound on the other hand is excellent and music fits the intense shooting action perfectly. I still like to play through the first level just because I enjoy listen the music so much. Sound effects are also great and all the time there's some kinds of explosions or voice acting heard through your DS's speakers. But I would recommend you to play Contra 4 with headphones on: it sounds that good.
Both the DS's screens are used in gameplay and it works just fine, although you might need some time to get used to it. As far as I know the touch screen is not used at all. Gameplay in general is that same 2D side scrolling shooting action you remember: shoot everything on sight and gather different weapons on your way… and lose it every time you die.
Contra 4 is a hard game: I've played on Easy and that's enough challenge for me. Pretty much the only way to make some progress is to know in advance every single enemy you'll face. This means a lot of trial and error gameplay, but since Contra 4 is so fun you most likely want to do it multiple times anyway.
The bad thing is that if you wish to play with a friend you'll both need to have your own copy of the game. This means that I probably will not be able to play with a friend, since there's no Wi-Fi option what so ever.
Contra 4 is an intense and classical experience, so don't expect to see anything new here in terms of gameplay or storytelling.