What are the four slots under emblem and badges

Solution 1:

These are to show your current level in all of the previous Call of Duty Games. Here is the order:

1st slot - Call of Duty: Modern Warfare
2nd slot - Call of Duty: World at War
3rd slot - Modern Warfare 2
4th slot - Black Ops

Also, if you have prestiged in any of the previous games you get a prestige token to use in the Modern Warfare 3 Prestige Shop (with a max of one token per previous game, there is nothing to be gained after the first prestige).

Solution 2:

These icons on your callsign represent your highest rank or level of prestige in the previous four Call of Duty games. They are for (from left to right):

• Call of Duty 4: Modern Warfare
• Call of Duty: World at War
• Call of Duty: Modern Warfare 2
• Call of Duty: Black Ops

The shape/style of the icon differs depending on what the highest level of prestige the player earned is just like they do in Modern Warfare 3, or just show the rank icon if you never prestiged in that game. The CoD Wikia has images of all the prestige icons that you can look at for reference. Suffice it to say that if someone has an icon in each of these slots, they've probably been playing Call of Duty for many years.

This is one of several features that were introduced in Modern Warfare 3 to honor long-time players of the franchise. In addition to an icon representing your prestige, if you have prestiged at least once in any of these games, you get an additional Prestige Token in Modern Warfare 3.

For instance, if you've prestiged 4 times in Black Ops and 1 time in Modern Warfare 2, you'd get an additional 2 tokens, one for having prestiged at least once in Black Ops, and one for prestiging at least once in MW2. Your callsign will also show the 4th prestige icon for Black Ops, and the first prestige icon for MW2.