Eliminating a 3 way switch
Start out by removing the 3-way you want to eliminate. Connect the wire from the black screw terminal or common to one of the two travelers that were connected to the 3-way; wire nut the other traveler. Now remove the 4-way switch and connect the two travelers from the 3-way you're keeping to the new 3-way you'll be installing there. You don't want to keep the 4-way there. Mark the two travelers with yellow tape. Take the wire you previously connected to the wire from the common terminal of the eliminated 3-way to the new 3-way black screw or common. Wire nut the other old traveler.

The vacated switch box will still have power if the common that was there was hot. If it's not, then the other original 3-way will have the hot and you can eliminate that 3-way if it works for you by following the same procedure

Again, this is how to do it but you have to make sure you're not in violation of code.