It isn't "dangerous," but 4 tires of the same brand and model are better to have, for a number of reasons. They "wear out" at the same rate. They have the same tread pattern, and therefore the same traction characteristics, etc. Having the two tires that match as the "drive" tires (being the front on your dad's minivan) is the best bet, the back two tires basically hold the back of the van off the ground.