I tend to think that 4 = still a toddler, and there is really no need for them to have a DS. They are not emotionally prepared for real responsibility or accountability at that age ...
I would think that a LeapSter would be more age appropriate.
Whatever you do, PLEASE don't be one of those parents who allows a DS to become a 'virtual pacifier' ... if you are at a public event such as a concert, movie or play and they need to play WHILE the event is going on, they are obviously too young to be there and you are being very rude and showing poor parenting by allowing it ... same goes the second you allow them to walk around a public place where there are any amount of people ... or at a library, etc ...