Sorry - should have made it clearer - you jack one side up at a time!!
Thinking back, I used a block of wood in the door jamb to keep it open if I was welding the floor, putting in a new clutch cable etc etc. Used the same block of wood when I worked on Mini's as well so I'm afraid that the wood was not R4 exclusive!
You'd need to weld in some of those metal strips you see on new cars. They are bent with a "hump" that slides over some other piece of metal (the strip itself is pushed to that piece of metal). Those strips tend to stick from the body into the door and they rotate along with the door when you open it. They provide just enough friction to have the door stay open when you open it. Though I think you might have to use some pretty heavy duty ones since the doors on an R4 are quite leaning inside, which will have gravity working against you pretty badly when you want to install such a system. Can't think of any easy solutions.