My Dewalt d26541 orbital sander, which I have had for 4 years, stopped running. Could the brushes cause this or is the motor shot?

Brushes can definitely cause this but unless you feel fairly confident in pulling the unit apart, it may be more cost effective to just replace it. It would be unusual to see the brushes wear out so quickly but I do not know how much you use it or the load you put it under. I have a Rigid that I use a LOT and have had it for probably 5 - 6 years and it is holding strong. These things just tend to last. I would contact DeWalt and see if you can have them send you new brushes first and then go for it; the worst is that you ultimately have to replace it (which you might have had to anyway) and the upshot is that you fixed it yourself and now have many more years and sawdust sneezes in your future.