Divergent Read-a-long

I always wanted to join a Read-a-long and when I found this one I immediately joined. They were reading Divergent, my new favorite book! Sadly, I joined after the first Discussion.
This read-a-long is hosted by Evie @ Bookish and Rachelia @ Bookish Comforts

They do a recap of the chapters read every week and then post questions about them. We answer them on their post but I wanted to post my answers here as well. The real discussion this week is @ Bookish Comforts.
Here's the link to their Discussion # 2, chapters 10 - 20
If you haven't read the book, stop here because there are spoilers ahead.

I've already read this book twice so I know all the answers but I try not to get ahead of them. If I had joined their read-a-long from the beginning I would have re read this book with them.

This week was harder to answer because I already knew the answers for all of them, so I tried to remember what my first reaction was to these chapters.

1) Why do you think there are guards along the fence that marks the city’s borders & that the gate locks from the outside? Do you think the gate is keeping people/things out or in? What are the Dauntless protecting or hiding?
I already know the answer to this but my first thought about this was, there's something dangerous out there and they're protecting them. Then I thought about it and wondered if there was something they were not allowed to see outside the fence (Were they trapped inside?)

2) Tris notices Four standing alone on Visiting Day & remarks that either his family doesn't like to come together or that he is originally from a different faction. If Four is from a different faction, which do you think it is?
I didn't know what to think of this, was he all alone or if Tris' mother recognized him, was he hiding something?

3) Why do you think Four said that he "[doesn't] make a habit of associating with the Abnegation”?
I definitely thought he was hiding something but I wasn't sure if I was right.

4) Tris' mom says she wants Caleb to research the simulation serum. Why do you think she is interested in the simulation serum used for the aptitude testing? I wont answer this one because I can't remember what I thought the first time.

5) Al says he chose Dauntless because he thinks "it's important to protect …" and "to stand up for people”. What do you think was Tris' main motivation for choosing Dauntless? Why do you think Tris's mom decided to switch factions and join Abnegation?
I think she wanted to be free, she did feel like she couldn't do much where she was, you can see that in the first chapters.

6) The simulations are all about "facing your fears". What is your biggest fear? Do you think you could handle the hallucinations as well as Tris did?
My biggest fear? I think I'd have the most fears out of everyone. I don't think I could handle them, that's part of the reason I thought twice if I could join Dauntless.