Aspect ratios 4:5 and 5:4 are basically the same, so Lightroom shows 5:4 when a landscape image is selected (the expectation being that the user would want to keep the horizontal edge longer than the vertical edge) but shows 4:5 when a portrait image is selected (the expectation here is that the user would want to keep the vertical edge as the longer). However, if you really do want a 4:5 crop on a landscape image you would use the 5:4 ratio, then flip it to 4:5 using the dedicated icon which I have indicated in this first screenshot:
After clicking on that icon, the crop guide switches to 4:5, and the ratio itself is changed from 5:4 to 4:5, as shown here:
That icon is the same on all versions of LrMobile, though it's location differs depending on the device being used. I've shown the iPadOS version, I think the iOS version has the icon top left, and Android has it top right.