Totally Reasonable Requests.
Great post - you've really laid out your situation well. You seem like a really respectful, thoughtful person, and it sounds like you're having some uncertainty in the midst of a really fun situation.
If the four of you are going to be fluid bonded, I would definitely say that it's beyond acceptable - it's imperative that y'all sit down and talk about sexual safetly/possibly sexual exclusivity as a group. It sounds to me like you're looking for a way to do this that is respectful and non-assumptive.
In my opinion, I feel that it's completely appropriate to ask if they're seeing other people. When we are fluid bonded with someone outside of our relationship, I ask for the courtesy of a heads up if they are sexually active with anyone else. I let them know that we will do the same. I also state my intent to be exclusive when/if it's there, and ask where they are at with the same issues. If you want to do this without being assumptive, I would say, "It's important to me that you know that we're not making assumptions." Be honest about that integrity inside of yourself; it's a great quality!
To me it sounds like your challenge is HOW you're going to ask this question. Given that you're a thoughtful person, you want to deal with your own concerns, but be respectful of their autonomy as people too.
So, my question for you is how are you going to ask this question, and how are you going to ask for the security that you need? Practice here! It's a great sounding board.