Start with the two shapes
A 4000 × 3000 landscape image has a 4:3 ratio. A 10 × 8 inch frame has a 5:4 ratio. The image is proportionally wider, so it cannot touch all four edges of the frame while preserving its shape and every original pixel.
The mismatch does not mean the source is unusable. It means the composition and the border are part of the design. Compare both outcomes at the same target size before deciding which aspect of the image matters most.
Fill the frame
To fill the 10 × 8 frame, retain 3750 × 3000 pixels from the 4000 × 3000 source. This removes 250 pixels of width, or 6.25% of the image area. A centred crop removes 125 pixels from each side and produces 375 effective PPI.
Move the crop toward a person near the left edge and more of the discarded width comes from the right. This can improve the composition without changing the retained dimensions or PPI. The frame shape sets the amount of the largest possible crop; position decides which part you keep.
Keep the whole photo
Fit the complete source into the same frame and it occupies 10 × 7.5 inches. The unused half inch of height becomes two quarter-inch borders when centred. All 4000 × 3000 pixels remain, and the photo supplies 400 PPI.
The border is a real part of the final layout if you include the complete frame in an export. If you export only the photo pixels, the surrounding canvas is omitted. Decide whether the receiving application should see a finished framed rectangle or just the image itself.
Choose based on the content
Fill may work well when the subject has room around it and the edge-to-edge presentation is important. Fit may be better when a signature, a group, or another essential detail reaches the edge. There is no general rule that the highest PPI option is the better composition.
Look beyond the centre. Check hands, feet, faces, text, and horizon placement. A crop that removes only a few percent can still remove exactly the detail you care about. Keep an additional allowance if the frame lip or a print provider may obscure part of the edge.
Export the choice you actually made
For a cropped photo, use an image export and check the downloaded pixels. For a page with a specified photo size, margins, or repeated copies, use the paper-layout PDF. Print the PDF at actual size so the physical dimensions remain meaningful.
Keep the original file available. Exporting one crop does not need to overwrite it, and a future frame may call for a different shape. Saving the layout parameters helps reproduce your decision, but a parameter-only project still needs the original image to restore its preview.