Intercepts a blue ball if a four bit register equals 15. Otherwise, a red one.
To make sure I understand, you have a four-bit register (laid out vertical or horizontal?) and after you press a starting lever, balls run through your layout, perhaps changing the register. But after the computation is complete, the register is returned to its initial configuration, and an interceptor holds either a red or blue ball, depending on whether the value is less than 15 (red), or equal to 15 (blue).
The register is free to change, and laid out in a direction of your choosing.