Complex sentence: It was finally rejected sometime after April 1981, originally a Fourth Doctor submission, but it is not known if the idea's development extended to the point that Greer modified it to include the Fifth Doctor.
Split into simpler sentences: It was finally rejected sometime after April 1981 and was originally submitted featuring the Fourth Doctor. It is not known if the idea's development extended to the point that Greer would have had to modify it to include the Fifth Doctor.