Agile doesn't ship things every 2-4 weeks. People have the most bizarre conceptions of what agile is.
There are iterations, typically focused on small set of features. The goal is typically to be 100% done with those features. Scrum, in particular, typically has the goal of bringing those features to shippable levels of usability and quality. This does not mean you will ship at that point! The ideal is that you could, at the end of any iteration, choose to ship what you've got because what's done so far is at a customer usable level, but the reality is you won't because it's not a viable product as a whole.
This is really not terribly different from heavier weight iterative methodologies introduced 30 or 40 years ago (agile isn't really new anymore either, being 15+ years old). The point is iterations focus work to completion on areas of functionality. Waterfall was a mile wide, inch deep at each stage: all requirements, then all design, etc, with nothing working until the end (which is actually a bastardized, crippled version of what Winston Royce actually advocated...see my earlier post).
Iterative tends to focus on the opposite approach, creating a very small set of working features, then moving on. Doing so uncovers a lot of risks in the first couple iterations: performance issues, hairy design problems, tricky network setups, etc. That's the big goal, finding out those nasty items up front whereas with waterfall you rarely find those out until the "big-bang" integration at the end.
It's also worth stating that agile really does not mean "coding faster". I've heard that so many times: "With agile, we'll get things done so much faster!". God no, that's not why iterative methodologies came about. The point is early insight into risks and early feedback on the items completed each iteration. Because of those things, you might very well end up being done sooner because you caught bad stuff early (either design problems or requirements problems), but you're not writing code any faster.
Nothing is going to take a 12 month project and magically make it 6 months. As Brooks said nearly 40 years ago, there is no silver bullet.