Assuming you've managed to create the new layer (Layer>Duplicate Layer), then just look to the right of your CS4 window, where you can see the tools palette. Here, select the Flood fill tool (looks like a tilted bucket) by clicking and holding the small arrow at the corner near the Gradient fill tool (which is selected by default). Then click in the foreground colour selector at the bottom of the tools palette and paste in the color code in the box at the bottom with the hash next to it. Enter only the figures, not the hash itself.
Now select the duplicate layer (Background Copy, or whatever name you gave it, by clicking on it), in the Layers palette on the right of the CS4 window, and click the image to fill the layer with the color you need.
Now at the top of the Layers palette, there is a drop-down with the word "Normal". Cllick the arrow and look in the list for "Linear Dodge (Add)" and click on it.
That should do it.