Azumanga Daioh is indeed in 4-koma, aside from the specials, which have parts done in more traditional comic style. A similar comic is Yotsuba&!, but it's not exactly the same. It's especially nice if you like iyashi-kei or "healing" genre, and it is a charming look at a slower pace and a delightful cast of characters.
I recommend Azumanga Daioh without hesitation. Plus, ADV Comics did a great English translation that preserved the humor while still making some of the very, very Japanese jokes and punchlines more accessible to a non-Japanese audience. I say this because some attempts to do that very thing have just led to weird "powdered doughnuts" situations, but this was far better handled. It's definitely worth checking out!