I am glad that you and S are more on the same page now. I do suggest that however the relationships with L and B develop - that you all sit down now.
If they know nothing about poly it seems like a good idea that they figure out fast what it means (to you, and what it might mean to them). In this case it's probably wise if you and S sitting down with them and say "If anything is going to happen, we need to talk about time management and boundaries (PDAs in front of kids, keeping sexual activity in the bedrooms and not on the couch where a kid or you could wander across it unexpected at 5 am, specific safe sex agreements that you and S have, as well as what L and B's safe sex agreements are, levels of disclosure if L or B take other partners and vice versa, how you and S define poly and how it fits into your lives, etc). If they can't do this, then I would express discomfort at moving ahead (well I would say we can't move ahead until you guys know the answers to these questions, but that's just me -ESPECIALLY safe sex agreements though, I am continually surprised how many new to poly people haven't even thought about it beyond "oh I guess we use condoms if there's PIV").
For me at least, something so complex as you have going on there, I just wouldn't be comfortable if any of the 4 of you can't sit down and objectively, maturely, openly and honestly talk about that stuff BEFORE getting any further into a sexual or romantic relationship. Even if S forms a sexual relationship with them and you don't, or don't until later, this situation affects you enough with them in the same house that I think a higher level of communication is called for than if they lived somewhere else.
Happiness will never come to those who fail to appreciate what they already have.