My advice is probably unique given that I lived with my husband and my boyfriend E before my hub and I opened our relationship. I'm basically starting my relationship with E after living with him for almost two years already.
I've only been at this a month, but here's what I've noticed so far:
Best way to avoid jealousy is honesty. If you all move in and realize the situation strains some boundaries you all may have set, it may be a good time to revisit the topic and come up with ideas to combat the difficulty of "hiding" your guys' dates. Be honest about your wants and needs, and I'm sure you guys can find a happy compromise that works. ^_^
Since you have a fair amount of life changes that are going on be prepared for some adjustment issues. Not only are you getting used to one person's idiosyncracies (the ones that only come out in a room mate situation), all of you are adjusting to a four-person scenario. A big house will help, but the living spaces you still have to share will bring out your differences if you don't plan for them. Talk about each other's cleaning habits and daily behaviors to start the process of compromise now, else it might be a bit overwhelming to deal with all of it after you move in.
Four months may or may not be too soon. How long have you known your boyfriend? I've known mine for over five years, so that can make a difference too. I would be wary if you don't know him that well.
Also, in the short term you can hide your lifestyle from your families by justifying the living arrangement to a bad economy....but in the long term it may raise questions. Also, what if someone moves out? Can you afford the raise in living costs if that happens?
I wish you luck in finding a living arrangement that is fun and beneficial for your family...I wouldn't rule out your idea, but if you need more time to work out the kinks that certainly doesn't hurt either. =)
Me: K, female, 27. Married to Y for over 4 yrs (male, monogamous, 33). Opened relationship to E (male, monogamous, 27) in a relationship vee.