Sounds like the lights on the bag are dual function, i.e. two separate lights inside the same housing. I would assume that the four wires are in two pairs: red & black, and green & brown.
The green & brown pair taps into the rear turn signal on that side, the green being positive and the brown the negative, to make the bag lights function as your rear turn signals.
The red & black pair from both sides tap into the brake right, reda positive and blacks negative, to make them both come on together as brake lights.
Alternately, if you would rather have them on all the time as running lights instead of as extra brake lights, you would tap into the rear running light of the bike, reds positive and blacks to negative.
I would recommend that you wire them as brake lights- they will realy catch the eye of the person behind you, where as if they are on all the time as running lights they tend to make the single brake light less obvious. Up to you though.