4:5 exists because the feed gives a portrait image more vertical room than a square but stops short of full portrait. Meta's own image ad spec publishes it outright: ratio 4:5, resolution 1440x1800, minimum width 600 px, minimum height 750 px, maximum file size 30 MB, JPG or PNG, and a stated aspect ratio tolerance of 3%. The arithmetic behind those numbers is simple — at 1080 wide, 4:5 is 1350 tall; at 1440 wide it is 1800 tall — which is why 1080x1350 and 1440x1800 are the same picture at two weights. Two practical consequences follow. First, 4:5 sits inside Instagram's published organic window — 1.91:1 up to 3:4 is a height/width range of 0.52 to 1.333, and 4:5 is 1.25 — so a feed post at 4:5 survives uncropped, while a 9:16 crop at 1.778 does not. Second, that 3% tolerance is doing real work: an image that is 4:5 plus a few pixels of slop still passes, but a 1:1 square that you “stretched” into a 4:5 box is not a 4:5 image, it is a distorted one. Crop to 4:5, then resize down to 1080x1350 or 1440x1800 depending on whether the destination is organic or paid.