M.H.
The only thing I would say is do you know 100% that your son won't just sleep through your 4 month old's crying? My son is not blessed to have a sibling yet (hoping, but no luck yet), but whenever he is asleep in the car and a younger child is screaming/crying, he stays asleep. I would have NEVER guessed he could do it, but if your house is somewhat soundproofed, your other son may never even hear his brother.
Again, maybe you know your 4.5 year old cannot sleep through it, but have you tried yet? Other than that, I really don't know...we did CIO with our son at four months old (which I have to agree was the best decision...he sleeps SO well!), so I really don't have any other tips. Good luck!