Stef, if I had to guess, since about the year 2000, there are literally millions of web pages, blogs, discussion forums, etc. that have discussed topics very closely related to your question. These include questions such as:
"How many megapixels do I need to make an X-by-Y print?",
"How big a print can I make from my X megapixel camera?",
"Should I up-rez the file myself before sending it to a commercial printer?",
"What are the best methods for up-rez'ing?",
"Should I try to sharpen before up-rez'ing?",
"Should I sharpen after up-rez'ing?",
"How should I allow a viewer to get to my X-by-Y print?",
"Will printing on canvas help?",
"Do the answers to any of the above depend on the subject matter?",
"Just how good of a camera / how good of a lens do I need to get a x by y print?",
"At what size would I start benefiting from mounting my camera on a tripod when I know I'm going to print large?"
"What quality factor JPG should I use to print this size?",
etc.
Obviously, no single post can address all these aspects of what is essentially the same problem.
However, let me point out a few things:
1. If you are uncertain about whether a particular file will print well as a big X-by-Y print, have a couple of smaller prints (say, 8x10") made at the same magnification. I would suggest one print from the center (or main subject) of the photo, and another from one of the corners of the print. Tape them up to a wall and view them from the same distance your viewers will see them.
Every image (and subsequent processing) is different, and having a couple of smaller representative prints (as described above) made will give you a very good idea of how the large print will look, but at a much lower cost to you.
2. Of all possible subject matter in the world, trees with branches and leaves need higher resolution than almost any other subject.
3. Everyone has a different threshold when it comes to acceptability of a print from an image. I can guarantee you that many people would be quite happy with a 24x32" print made from your 4Mpix image. Other folks (including myself) wouldn't dream of trying this, even though I have some of the best production methods at my fingertips.
4. An old photographic trick is to up-rez your image to very safe & appropriate pixel dimensions for a 24x32" print, and then, intentionally introduce some high resolution grain / noise once at this resolution. The viewer's eyes focus on the sharp grain/noise and this makes the overall image "seem"sharper to casual viewers.
5. The same idea (as #4, above) applies to printing on canvas, and/or introducing other sharp distracting elements in your up-rez'ed version, eg, luminosity thresholding, half-toning with Ben-Day dots (a la Roy Lichtenstein), fake borders and matting, over-the-top colors, etc. Some of these distractions won't be to everyone's aesthetic taste, but they definitely do work in making an image with marginal resolution seem sharper.
HTH,
Tom
