I have just noticed that there are reverse thrust sound parameters for only two of the four engines on the 707. I thought that all four engines had thrust reversers?

I guess it must have thrust reversers on all four engines, because the vents open up on all of them when this is activated....

All of Boeing's commercial airplanes, from the Stratocruiser present, have had reverse thrust capabilities on all the aircraft's engines. The only modern airplane I know of that has reverse thrust on only two engines is the Airbus A380, which has them on the inboard only.