Yeah the Knife of Dunwall is essentially a smaller version of the original. The gameplay is solid though the story is a bit weak. Just like the game. So worth $4? Sure. A good game in many ways made better with improved enemies. Just savor the game and don't bulldog through it.
Dishonored itself is definitely worth the Steam Sale price! It is one of the better single-player "first person RPGs" out recently. The mechanics are neat, there is a ton of freedom of choice (though, there may be consequences for some of those choices), and the world/lore/story is interesting and well though out. It is sort of like a "steampunk Deus Ex: HR".
"Voidwalker's Arsenal" DLC is worth the price, especially on sale. Buy it when you buy the main game as it enhances the main campaign! It gives you a considerate amount of bonuses and upgrades, including "bone charms" which act as unique passive buffs. Its a great little bonus to start the game with, and for a few dollars you get all the stuff that used to be pre-order retailer exclusive!
The "Knife of Dunwall" DLC is definitely worth the price. Play it AFTER you play through the main game; its spoiler-filled. The story provides a look into things from another perspective, that of a prominent "anti-hero" and/or villain in the main campaign. The DLC is like others have mentioned, like "more" of the campaign, which in this case is a good thing.
The other DLC, the "Dunwall City Trials" is hit or miss - if you like time-trials, inventive scenarios and "mini-games" of sorts, you'll enjoy it. There are lots of tasks here if you're into it, but it doesn't play into the main campaign.
Overall, I say on sale, Dishonored itself is an excellent purchase if you enjoy the gametype. If you buy the game, get the Void Walker's Arsenal and Knife of Dunwall DLCs on sale too - they'll definitely enhance your experience.