 According to three-dimensionalism, the thing that was doing whatever you were doing at noon yesterday was you. It was you who was doing that, and now you are doing something different (namely, reading this sentence). So the relation between “you then” and “you now” is identity. According to four-dimensionalism, on the other hand, the thing that was doing whatever you were doing at noon yesterday was an earlier temporal part of the thing that is you, and the thing that is doing what you are doing now is the present temporal part of you. The relation between “you then” and “you now” is the temporal counterpart relation. (This is similar to the relation between your left hand and your right hand, which is the spatial counterpart relation. Your two hands are distinct parts of a bigger thing that contains them both.)