Let's party like it's 1987.
Konami has stayed true to its roots in this iteration of the series, fortunately. Arcade mode is not different from any previous games in the main series; in fact, some music, bosses, levels and enemies were ripped from older Contras. Nostalgia anyone?
Despite the simple concept and an arcade mode that only consists of 9 levels, Contra 4 is far from being a short game, and its learning curve isn't necessarily immediate. Later boss levels, 3D missions and a unique ocean level offer different tactics and sometimes controls, which ultimately makes the game better. New weapons affect the game largely (the pea shooter barely deals any damage, which forces players to constantly be on the lookout for better weapons - what makes the game very hard is that if you die, you lose your upgraded weapon), and the variety is good. And if you haven't realized by now, the Contra series is hard. This is perhaps the hardest in the series, which means you'll be re-trying the arcade hundreds of times to get past normal mode -- making the mileage for your money pretty high. Multiplayer requires two carts, but if you find a friend with a DS and a card, get ready to kick ass on a game that seems much easier than its single player counterpart.
What makes Contra 4 superior to most other DS platformers, though, is not just in the difficulty it boasts; incredible boss fights, exceptional level design and a great soundtrack make Contra 4 a winner. You will learn to interpret "boss fights" as "adrenaline rushes;" they're creative and the incredibly fast pace is stunning. The level design here is very interesting; whether flying on the outside of a rocket or taking control of an enemy ship, the game always has insane ideas to throw at you. The soundtrack by freestyle composer Virt is great; re-vamping older Contra tunes works very well, as one would expect.
Bonus features are plentiful and extremely entertaining. Fun, short missions with varying goals - and great rewards. Contra 4 is an arcade shoved on a portable system; it's fun to be able to pick it up and play through the Challenge Levels on the fly (beat the game on easy to unlock the challenge levels). Their short length makes the game much more portable.
Overall, a rewarding game whether you played the older Contras or not. If you're casual about your gaming, I'd suggest you avoid this; but if you're anyone else, this is an excellent game in an excellent franchise.