Ah, OK. A four treadle loom is probably what we call a "direct tie up." In other words, there are four harnesses, and each one is connected to one of the treadles. Depending on whether the loom is a rising or sinking shed, stepping on a treadle causes the corresponding harness to go up (or down) and carry the threads that pass through its eyes with it. To raise (or lower) two harnesses at once, you step on two treadles at once. Eminently simple, but it does limit the patterns that can be woven to some extent.