nimoy didn't write ST: 4, just directed it. he may have had the idea, but didn't write it.
http://www.imdb.com/title/tt0092007/fullcredits#writers

as for ST: 5, shatner had a "well if he could do it (twice) then i could do it" moment with directing it, but again didn't write it just had the idea.
http://www.imdb.com/title/tt0098382/fullcredits#writers