That looks like the right cable. USB from the computer to male DB9 RS232 (to plug in the female DB9 on the inchworm).
That will allow you to run the inchworm or other serial devices via a USB port on your PC/laptop.
It will still be slow, but you can use it on a laptop as well. The 4 feet is not an issue as it will be USB and get converted at the DB9 end. Will look again. But I think you are find with it.