It's the same 4 directions as shiho giri as David explained, but NOT as he explained in terms of 4 different attackers.
Essentially, you can throw (the same) uke in the opposite direction of the attack, i.e. behind him, in the same direction as the attack, or to either side of the attack (from the same side that you are on).
In actuality, it can be in *any* direction... probably easier to show than explain in writing...
It is also used to practice turning to receive another attack from another direction after
the throw of the first attack because your attention should not be on the person you have just thrown but possible attacks from other directions.
None of the techniques of Aikido are practice to do just one thing. When practicing a technique you are practicing multple skills.