Okay, so your 4 year old is close enough to 40# that there's no point installing with the lower anchors. This is unfortunate as the lower anchor centre install is offset to the driver's side which makes it possible with some seats, to be able to access the 3rd row.
And it's almost certain you'll need the harnessed seat in the centre when your son is in a booster. I'm on my phone so hard to type all the details, but it's unlikely a booster will work in the centre seat. I'll provide the reasons when I'm on a real computer and have more time.
You've got 2 options really, for the middle row.
1. Narrow harnessed seat in centre with seatbelt, booster outboard driver's side, should be able to access 3rd row hopefully by sliding seat past centre child seat
2. Harnessed seat of any size outboard driver's side with seatbelt, booster outboard passenger. Then pop booster out when you need access to 3rd row and an adult sits in centre between 2 kids. I wouldn't recommend this being a large adult.
Now, maybe there is someone who has successfully used a booster in the centre. If so, hopefully they'll pop in. It would have to be narrow and you wouldn't be able to use the lower anchors if it has that capability (due to how the seatbelt and anchors overlap). Could also be an issue if the booster requires head support as the centre headrest only goes up so high.
You'll need seat recommendations of course, but I just wanted to throw out this info while I had a chance.