Originally Posted by
thumpism
SunTour Perfect 5-speed freewheels also thread the last two cogs and should still be available. I have several. Lose the smallest and ride.
I don't believe a Perfect or ProCompe 5 speed body would work to create the OP's 4 speed freewheel. The two smallest sprockets both thread to the body. Thus the body is as "tall" as the entire stack of sprockets. This is also the case for early Shimano 5 speed freewheels.
From left to right: Perfect/ProCompe 6 speed standard body, 6 speed Ultra Spaced body, 5 speed body. It can be seen that in the case of the 6 speed versions the added sprocket is a splined and not threaded version.