How can I eliminate some of the switches in a 4-way circuit?
Right box

Connect the black wire from the power source to the black wire on the cable going to the middle switch box (this will be the hot wire to the new switch).

Connect the black wire from the lamp to the red wire going to the middle switch box (this will be the switched hot from the new switch). 

Connect the white wire in the cable going to the middle switch box to the two white wires already bundled. Remove the black marking (this is now a neutral going to the new switch, even if it is not used).

Middle box

Connect the black wire from the right box to one pole of the switch, the line side if there is a distinction (this is hot).

Connect the red wire from the right box to the other pole of the switch, the load side if there is a distinction (this is now the switched hot).

Leave the white wire capped unless the switch needs it. Remove the black marking (this is now a neutral to be used by a smart switch if needed).

Cap all the wires going to the left box.

Left box

Cap all the wires.