Joker,

If I am understanding your question, you want to print a picture that is as large as 4 sheets of letter size paper... I'll assume that you want each of the 4 sheets of paper oriented in a "landscape" position. This will allow you to piece 4 separate 10" x 8" images together to make a single 20" wide by 16" tall image.

If you are using Photoshop, you need to go to the image size window and check to see that you have at least a 20" x 16" image at a minimum resolution of 150 dpi. You will then go to the image canvas size window and "crop" each corner to a 10" x 8" image and save them each as a new file.

Print each 10" x 8" file and then paste them together...

Dan J