4 horses are walking in the direction of a moving train.

Solution

There are three partially black and one fully black horse. This is possible, as it satisfies the condition of one black horse and three white horses. There are 3 white horses and one black horse. This is what the passenger in the train observed, so it must be true. All four can be partially white and partially black. This is possible as well. All horses can have a combination of black and white color. There are 2 black horses and 2 partially black horses. This cannot be true because the observer in the train clearly saw one fully black horse and three white horses. Therefore, there cannot be two fully black horses. So, the statement that can never be true is option 4: "There are 2 black horses and 2 partially black horses."