These are redstone comparators. However, because minecraft doesn't measure a currents intensity I made it so the right most input was the deciding factor on the output.

Explanation: These circuits will always give only one output. It will always be the right-most output that is one. So if you have a row of 4 switches you can turn on each one individually and their individual output will turn on individually. But if you select input 1 and 3 then only output 3 will be active. If you then select input 4 (so now input 1, 3, and 4 are active) then only output 4 will be active.

The circuits include one with 2 inputs, one with 3 inputs, one with 4 inputs, and one with 5 inputs. The five input version should be stackable. (not tested)