Just Dance 4

While it sounds like this is going to be exactly like the Sweat Challenges from Just Dance 3 and JD Greatest Hits, it is quite different actually.

First, in order to burn calories in Sweat Mode, you have to choose Just Sweat from the "Main Menu" and then pick a sweat class. While you are doing the class, it will keep track of the calories you are burning.
The classes themselves alternate between an aerobics/dance routine and actual songs from the game's track-list. Unfortunately, your results from the Sweat Classes are not carried over to your results in the "Just Dance" list (i.e., you can't use "Just Sweat" as a means to 3 and 5 star every song in the game).

Second, completely unlike JD3 and JDGH, you don't have to burn all 2000 calories in one sitting. JD4 keeps track of the calories you have burnt over your "career" in the game. To check your progress, click on "My Card" in the Just Sweat menu (i.e., instead of one of the classes), and it will give you information pertaining to how long you've spent in sweat mode, how many calories you have burnt in your career and how many calories you burnt in your most recent sweat class.

Third, once again unlike JD3 and JDGH, JD4 does not count the calories that your partner (if you had one) burnt. It will only keep track on the calories that you burnt (or player 1 burnt, I guess). While you are performing one of the classes with a partner, it will keep track of their calories, but it will not add them to your stats.

Fourth, you do not need to do the whole class to have your calories tracked. You can quit at any point in the class and your calories will be added to your stats (again, which are displayed in "My Card"). Also, for me the achievement for 500 calories popped mid-song, so if you find yourself a few calories short of a particular "sweat mode/calories" achievement, just start a 10 minute class and you'll probably have the achievement within a minute or two (that's actually what I did for the 500 calorie achievement).

Well, that is all you need to know about the achievement itself, another thing that is worth noting is that JD4 doesn't seem to be as generous with props as JD3 of GH were regarding calories/sweat points. However, if you are lucky enough to get a towel/hoodie to be read as a dancer, you will get a few more calories than if the game was tracking your calories (i.e., if you were actually dancing). Again though, if you do get a towel or hoodie tracked as a dancer, make sure that it is read as the dancer you are trying to burn calories with.

Finally, about the classes themselves. Before you start a sweat class (there are 10, 25 and 45 minutes classes for each of the 5 routines), it will tell you how many calories you should expect to burn in the class. I have found that their estimates are way off and I always burnt far fewer calories than they told me I would. All in all, unless you figure out a trick I didn't, expect this achievement to take about 9-10 hours in Sweat Mode.