So, when I render out 2k, 1080, 720, etc. it scales everything properly.
But when outputting 4k, for some reason Da Vinci doesn't scale properly, with "force sizing to highest quality" turned on or off. It's a mix of 5k epic footage, 4k DPX FX, and 4k .r3d footage in the timeline.
But when I render it to 4K DPX, it comes up with both letter and pillarbox.
Any suggestions for a suggestion I might be missing?
For the record, I'm using a custom 4k setting (4096x1714, the DCP 2.39x1 setting), but it seems to have the same behaviour when working with the 4k default setting.
Except that 1920x1080 is 16:9, and what you're now using is 2.39:1.
What sizing option do you have selected for fitting the material into the frame? If you're using "scale entire image to fit" and you have a source that's taller than 2.39:1, it's going to pillarbox. If you have a source that's wider than 2.39:1 (unlikely, but possible) it's going to letterbox. That's how it's supposed to work. If you want to matte everything to 2.39:1 and fill that frame, you'll have to adjust sizing on shots that do not fit using the standard methods.
One thing you might want to try as a test is to set the timeline to 3840x2160. That's exactly twice the size of an HD frame and should give you the same relative sizing.