Answer:
1) The farmer and the chicken cross the river, (the fox and feed are safe together), he leaves the chicken on the other side and goes back across.
2) The farmer then takes the fox across the river, and since he can't leave the fox and chicken together, he brings the chicken back.
3) Again, since the chicken and feed can't be left together, he leaves the chicken and he takes the feed across and leaves it with the fox.
4) He then returns to pick up the chicken and heads across the river one last time.