For other people's experiences, which could be very helpful, you may want to read our discussion thread called "Multi-Partner Co-habitation" here: http://www.polyamory.com/forum/showthread.php?t=2349
Also, do a tag search or advanced search for the keywords "cohabitation," "living together," and "moving in together."
I don't know how long your hubs has been with his gf, but as far as you and your boyfriend, I think you will find that, in general, most people would say that four months is way too soon to be moving in together. It's often recommended that multiple partners wait at least a year. That way you can see what it is like to be in a poly arrangement through all four seasons, every birthday and holiday, and possibly even emergencies. At four months, you're really still getting to know each other - being able to live with someone is a whole 'nother ball of wax.
However, I think if it's a big enough house, and you all kind of treat it like a roommate situation or renting a summer share, maybe it isn't too soon. Anything's possible! I do think you are smart to ask as many questions as possible and try to figure out various scenarios, as well as give everyone an opportunity to escape and have privacy. Very smart!