DPI refers to what a printer uses as how it prints an image. The concept of DPI for the most part is past history as all the printers I tried at work (I work in IT at a college) they just ask how you want the image laid out and they don't care about the DPI. Modern Printers (their drivers) will scale the image to fit whatever output you want.

I tested an image that is 1280x720 and the original file says it is @120DPI. I took that image into Photoshop and it says it is 1280x720 @ 72DPI. I copied the image to a blank image setup as 400 DPI and then it says it is 1280x720 @ 400DPI. I exported that image and the export does not list the DPI. I reloaded the saved image and it is 1280x720 @ 72DPI. The pixels are the same just some vague settings of DPI that changes.

What is it that you do that requires the image to say it is 400DPI? Since the printers I have only have 2 DPI setting 600x600 or 1200x1200 your image is either going to be output as pixels on a 1 to 1 basis so would be 400/600 or about 2/3's the size ~ 5.3x6.6 inches or the printer will scale it to 8x10. As I said above DPI is a past concept for the early days of computers when they didn't scale images and could only output 1x1 so you had to make the images to fit the printer you were going to use as your final output device. If you have a printer that is 400 DPI then your image of 3200x4000 should fit properly.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13