I'm not a tech and only have 1 child, so I have no experience with attempting 3 across. Hopefully someone with more experience will have a bit more advice for you.
I will say that the Sunshine Kids Radian's are the most narrow, high weight harness convertible carseats on the market.
Also, I believe the Chicco Keyfit is the most narrow infant seat on the market.
Your 4 year old definately would be safer in a 5 point harness, as long as he fits in one, and it would be easier than having him try to buckle his seatbelt between seats. Remember, the bare minimum to use a belt positioning booster is 4 years old AND 40 pounds. Even then, most 4 year olds do not have the maturity to sit properly at all times in a booster.
Also, the most ideal condition would be to have your least protected child in the most protected part of the car. Should you choose to keep your 4 year old in a belt positioning booster, he would be safer in the middle of the back seat (assuming you can get all carseats installed properly. Should you decided to put your 4 year old back in a harnessed seat, then I would still try to put one of the forwardfacing children in the middle. Ideally (for safety) the smaller of the two forward facing children in the middle. But, if you are afraid of your 2 year old messing with the baby, you may want your 4 year old in the middle.