I assume that by "4 buttons", you mean that you have two coin mech switches and two reject switches shown above.
Wire the coin mech (red) and reject switch (green) in parallel so you can either drop in a token or press the reject switch to coin up.
Encoder ______/_________ Ground
| |
|___/____|
Left mech and reject for P1.
Right mech and reject for P2.
Scott