ETA: Somehow, I loaded an old version of this thread, so I missed the last few replies. Fixing what's already been addressed now...
The only spot to tether your 4yo is in the center of the 3rd row, so that's really where he needs to go. (The only other option would be to booster your 7yo outboard in the 3rd row and put your 4yo in the second row where there's a dedicated tether.) It's frustrating -- believe me, I know -- to know that sometimes the safe option is not the most convenient one. Moving the baby to the back would work, if you feel that that would be an easier option than putting your 4yo in the center of the third row.
(The Caravan isn't a vehicle where the tether is offset by use for either outboard or center, is it? I don't have my LATCH manual handy, but I know that's an option in some vans. I'll try to check later if no one else replies.)
I would say either a Nautilus or a Maestro, or a ProSport. The ProSport has some unusual rules/routings but as long as that doesn't bother you, I like mine a lot. The Maestro might be okay, if you plan to buy your 7yo a dedicated booster eventually and pass the Frontier to the 4yo when he needs it. The main reason I would do this is that getting a dedicated booster might provide a better fit on your 7yo than the Frontier, since he's so petite -- I often find I prefer the fit of dedicated boosters than combo seats.