So concerns are:
Physical safety, because 4 is still young enough to grab dangerous stuff that might be left around in a household where children don't already live.
Physical safety regarding this new roommate is unknown at this time. Do you have concerns there, if, say, their father is too impaired to be aware or if he steps out leaving the roommate with unmonitored access to the children?
Dad potentially being too impaired to parent.
I was able to address these points with my ex and get satisfactory answers regarding my kids' safety when they visited him at his various residences. At one point, he visited them at my house while I stayed somewhere else (I wouldn't do this again -- I have a visceral reaction to having him in my space whether I'm there or not).
I've felt pretty strongly that even though my ex is an imperfect jerk who was abusive to me that my kids need to know him and spend time with him regularly. He's never been abusive to them, and we live very close together, so the visits could be very brief at first. I have massive concerns about his effect on them... and I truly believe that not seeing him at all would be much, much worse. We are far, far from 50-50 custody. He's started to bring that up, and frankly I would fight it even now (they are 12 and 14).
Safety planning around a parent's substance use sucks. Is that something you can discuss with him about at all? Not the part where he should quit because he has a problem, but what safe parenting is (because I'm guessing that you've been the parent-in-charge while he's drinking and that's not going to work if you're not on-site). I'm mean this as an exploration, not as a directive, by the way.
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