You can do any size in PSE including 4x8. I use 2.0 and here's how I would do it. You may have to adjust this to fit 4.0.

1) Create a new file File->New. Put the following settings in the dialogue box.
Preset Sizes: Custom
Width: 4 inches
Height: 8 inches (note reverse height and width if you want landscape style verses portrait style)
Resolution: 300 dpi
Mode: RGB color
Background: white

2) Open the photo you want to use. Resize the photo to 4x6. Image ->Resize->Image Size. Change the width and height under "document size" to 4 and 6 inches.

3) Select the move tool and drag your photo onto the 4x8 document you created. Position the photo as desired. This should leave you with two inches for your greeting.

Let me know if you get stuck and I'll do my best to help. -Misty