I like Liam, Benjamin and Owen with your sibset. I like Liam because it has a different ending than Aiden albeit slightly, but I like the name Benjamin the most. I wouldn't worry about the initials BM, I can think of several people I know with those initials off the top of my head.
Re: help me decide!
Benjamin - definitely #1 in my opinion - followed by Owen, Liam, and Devon in that order.
2. Owen
3. Liam
4. Devon