4. Use an adjustment layer to make changes. In your Layers palette, it's the half-black, half-white circle, fourth from the left in the row of icons on the bottom. Click it and choose your adjustment. I'll use curves.
5. Make your changes, then click OK. Select the next layer, and once again draw a box around that layer's image using the Marquee tool. Then repeat Step 4-use an Adjustment Layer to do your tweaking. Keep going until you've done all four pictures. Then make a print. Take a look at your options, and pick your favorite.