RINNAI Water Heater Flush Procedure
From the pictures and the diagram, the best I can tell is V3 and V4 are the two valves with yellow handles further back on the sides of the two valve bases in your picture and they cut off the water inlet and outlet. V1 and V2 are the valves you have labeled as V3 and V4. They open the threaded ports so you can do the flush. A closeup picture from the right would help.