LT is our biggest need. But it's not our only need. Why pass up very good players at #4 that would settle our non-OL needs better over the long-term to take a guy that's only going to be okay for the role he was drafted, only to be transitioned to another position (that he's never played) in a year?
If we're in the OL or bust mindset: If we're going to draft Campbell with the idea that "if he doesn't excel at LT we can always move him to OG (where he's never played) in a year and hope he does well there", wouldn't be better to draft Membou with the mindset "if Membou doesn't excel at LT, where he has played (don't believe the people who say he never has, the videos are easy to find), he can always be moved to RT in a year.
If both Campbell and Membou fail at LT, isn't it better to have spent #4 on a stud RT, instead of a possible OG?