The differences are in the threads - read above...
Mainly the 401 and 500 have a cam stack the 403 and 503 do not. The 401 and 500 can be set to do the same stitches you can do with the 403 or the 503. When the 500 and 401 are set to do the stitches the width is set. The 403 and 503 have width control and you can set the needle left right or center when you use the cams. You can do all that on a 401 or a 500 if you use the cams so what advantage is there to have a 401 or a 500 other than you don't have to have all the cams? The biggest disadvantage to the 401 and 500 is the controls tend to gum up. If you get them ungummed then they have to have lots of TLC or they don't want to move. Those machines like attention. The 403 and 503 seem to be ready to go all the time. I guess I like simpler. I can also predict what stitch my 403 and 503 will make - the 401 and 500 not so much. You can set the dial at E and Q but if you put the lever at 2, 3 or 4 each position will give you some mutation and you would have to make a note so you can find it again. There really aren't all that many real combinations because they tend to repeat somehow. It is fun to play with the stitch selector on the 401 and 500 - gotta admit. But for dependable I like my 403.