instrument cluster display on 2005 Mercedes-Benz ML350

On my instrument cluster in the mileage display it shows a wrench, the letter D, the number 4, and a minus sign. I know the one wrench means minor tune up but what does the number 4 and the minus sign mean?

Asked by for the 2005 Mercedes-Benz ML350
Should be explained in the owner's manual. Check here or try this link you may be able to down load a free copy of the owner's manual for your vehicle
1 more answer
I believe it means you are 4 days over on the scheduled maintenance.