It isn’t possible for them to have the same amount of horses. Let me explain.
If we wanted to distribute 21 horses between the 4 of them we would have to do 21/4. This equals 5.25, and we know that they aren’t going to bring 5 horses and .25 of a horse; that’s wack!
One person would have to have 6 horses instead of 5. This is probably the answer your teacher wants.
If they really wanted it to be even they could sell the extra horse. Then they could have 20/4=5.
Or they could share that extra horse and send it from place to place.
Hope this helps you!