If you're looking to have them heat hot side and cool side, then with some trial and error you can get this, but as everyone else has pointed out, each size puts out a different wattage.

However, I always thought wattage was more according to width, not length. If it takes into account length and width you might be able to get away with it since your 4" is shorter... Hook them up and keep track of the highs and lows with some temp guns and let us know!