Are the pictures Raw or .tif? Raw would need conversion, but a .tif can be printed directly - you don't have to convert to .jpeg.
For printing to a different aspect ratio, you will need to crop the photo. In order to retain the most resolution, you need to do a little math (not difficult, though). Assuming your camera is a 3:2 or 4:3 aspect ratio and you are printing to a 4:5 a/r print, first divide the number of pixels on the short side of the picture by 4. Multiply that result by 5, and you have the pixel dimension that you have to crop the long side to.
As Frank says, forget DPI numbers - just print the cropped file, selecting 'fit to paper' (or whatever your printer equivalent is) in the print dialog, and the printer driver will adjust the DPI to whatever it needs.