1) The image improvement will mostly be seen in color prints since there is the addition of Orange and Green ink in the 4900. Both the 3880 and the 4900 have the same number of black inks. Depending on the subject of the print you may see a noticeable improvement in B&W prints due to the newer print screening algorithms.
2) Epson offers several papers in 17 x 22 cut sheet size and one in 11.7" x 16.5". If you wanted to print an image that is 16x20 you could easily use the 17 x 22 size. If you wanted a borderless print you would have to manually trim off the excess. The 4900 has a built in paper cutter that can cut the paper at the tailing edge at any length you need. It will not cut at exactly the length you specify - it leaves a small amount of blank paper leader. You could set up a custom paper size in the Epson printer driver to 16 x 20. You will end up with a print image 16 x 20 in size with a half-inch border on the sides and approximately 2 inches on the top and bottom. Again trimming is recommended if you want a borderless print.
3) It may seem like Epson sometimes makes marketing decisions in a vacuum. But the reality is the paper sizes they offer are somewhat based on feedback from users and professionals alike. Here is another example that I have yet to understand. Epson offers their most popular photo paper, Premium Luster Photo Paper (260) in 16 inch rolls but not in 17 rolls. It is as if they have totally overlooked the 4880 and 4900 owners. All their other papers offered in this size range are 17 inch.
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