I hope you decided to start Rob Gronkowski.
Even if Aaron Hernandez is in the game, Gronkowski is Tom Brady's favorite end zone target, even more so than Wes Welker. If you are in a PPR league, Aaron Hernandez has a little bit more upside because he is more of a hybrid TE/WR but Gronkowski is definitely the more valuable player. In the last 3 games this season he has scored 5 touchdowns. Gronkowski is on my fantasy team and he is quickly becoming the most consistent TE and a strong candidate for No.1 TE for the season. I'm surprised that you have both Jason Witten and Gronk... I would suggest trading one of them (preferably keeping Gronk) for a top 10 WR or RB if you need either.
Don't get me wrong, Jason Witten is a fantastic tight end and also a strong candidate for No.1 TE for the season but with Aaron Hernandez out (possibly out until week 7 or 8), Gronkowski has so much more value now.
Even when Hernandez plays, Gronk puts up stellar fantasy numbers. Go with Gronk every time. For Week 4, definitely go with Gronk. He is not only the safer choice (because Brady isn't injured like Romo is) but he is also the better fantasy option due to his red zone looks). Gronk will not let you down even when Hernandez comes back.
Hope this helped and good luck!