The image data from the camera is just pixels. Resolution only has relevance when you will be transferring the image to or from physical media such as film or paper.
When Photoshop gets an image that does not contain a specified resolution, it treats it as though it had a resolution of 72 ppi. Again, the resolution is meaningless at this stage. You can change the resolution to whatever you want in Image > Image Size, with resampling UNchecked. Alternatively, change the print dimensions of the image in inches (or cm) with resampling UNchecked, and the resolution will be automatically changed to correspond to the new print dimensions.
Your 1280 x 1024 image won’t fit precisely into a 4 x 6 image, because it’s a 5:4 aspect ratio, not a 3:2 aspect ratio. You could make it 4 x 5 inches, with blank borders filling it out to 4 x 6, or make it 4.8 x 6 inches and crop it down. Please don’t unlock the aspect ratio and stretch it to fit.
I wish I could conform to such an analysis but I’ve seen quite a few apps where dpi and ppi seems to mean different things to what is explained here.
For example, an export at 72 is nothing like an export at 300 and oftentimes juggling these can give very pleasant results.
