Crop to 16x20?

You specify this when you export. For example, assuming you are working in the cloud version of Lightroom, Select the image you have just cropped to a 4x5 ratio, then select export from the file menu or click the share button (the square with the arrow going out the top) and select export. For File Type, select JPG, and select custom dimensions. For the long size, calculate the long side of your print times the resolution you want the print to have. This is typically 240 ppi to 300 ppi, so for example a 20" long print should have 20*300=6000 pixels on the long edge. Export your image and you should have a file that you can send to a photolab to print at good quality on a 16x20 size.

The instructions on Lightroom Classic are somewhat different but follow the same principle.