Hey H.!
My boys are 3 1/2 years apart and have shared a room since they were almost 4, and the other around 4 months, even though we have an extra bedroom and could've let them each have their own room.
Some reasons why we chose this are
1) My husband and I wanted and needed to sleep (really sleep, I'm not even talking sex here, although that was easier, too, since we weren't so stinking tired) together, without kids -haven't learned how to follow this with the dog, though!
2) I've read that one possible aid against SIDS is hearing the breathing of others to help the infant to remember to breathe while they sleep, so I was reassured that this was good for the baby
3)Their whole lives they're going to have to adjust to others, so the time it takes to get used to being in with someone else and learning how to not wake the other one up or to get themselves to sleep is good practice in flexibility and self-regulation
4) We live and work on a college campus and have discovered through the students' funny stories that kids who share rooms at home in general adjust much better to roommates in college - they have better expectations of how to live together and better boundaries
5) Speaking of boundaries, it's good for all four of you to know what belongs to whom, so it's healthy, normal, and beneficial for you guys to all have and use your own space. Someday you'll laugh that your tiny baby got the master bedroom and you and your husband slept in the livingroom! :)
You guys sound like a fun family! Good luck!
S.