I appreciate all the help. There are a few things I'd like to clarify. The second lightest child is also the oldest. Some have said the lightest/youngest kids - it kind of has to be lightest OR youngest. My DD is oldest and definitely tallest, but is second lightest. I'm thinking the two 3yo kids weigh the same, just under 40lbs.
DD does have a medical condition that could prevent her from being in proper position. She also severely lacks the impulse control to sit properly in a booster. Plus, the FR in booster mode is the only booster I have access to and it has a 40lb minimum which she does not meet.
As far as making my own kids safest, I disagree. All kids in my van get equal consideration in who gets tethers. That's why I asked for help.
Also, there's a chance I may only have 3 kids. The 4th (one of the 3yo) will only be with me if his mom goes into labor which in this case, is an emergency (I am their only option for taking him).
I'll play around with it tonight. Hopefully I can leave one seat in the off-set LATCH position and just do one untethered, only using that seat if I have all of the kids.
Oh and all of the kids besides my DD are about the same height, give or take an inch or two at most.
Again, thanks for all of the insight. I am still undecided. There are several good points made in this thread that have different solutions. I will have to give it more thought.