The Midichlorian idea was dropped long before the final draft of Ep4. You can clearly see from this that they had nothing to do with if you could become a Jedi or not. He was still following the plan of "anyone" could learn to use the force. The force in the OT was spiritual and not a medical condition. He only reintroduced Midichlorians as a measuring device to determine how powerful Anakin was in Ep1.
There are bits in there that made their way into the final version of the prequels.