-9 - (-4) is the same as -9 + 4 because subtracting a negative is the same as addition. I'm not quite sure how to represent it with the integer chip model, but basically, taking away -4 from -9 is the same as adding positive 4 to -9.
Perhaps draw the 9 negative chips and place the four negative chips under it, then cross out those that correspond to show that it is increasing in sum?