how to export as tif with 400 dpi from substance designer?

PPI (not dpi which is a printer property used to form pixels on paper) is just metadata stored alongside the image and tells a print driver what physical size to print an image - in other words how to convert a pixel size to a physical size. As such, for textures it is irrelevant. It could be 1ppi or 6000 ppi - it makes no difference to the quality of the texture. All that matters is pixel dimensions.

Note - Photoshop also assigns 72ppi as a nominal value when no ppi data is present at all.

That said, if you are going to print the output textures and do need this print metadata value to be 400ppi then you can simply open the image in Photoshop and change the ppi value to 400 (with resample unchecked) and resave.

Dave