I have a 2003 Grand Caravan. I have a Radian for my 5 year old that I love. It is installed with LATCH on the driver's side captains chair. I find it super easy to install with LATCH either FF or RF and with the seatbelt RF. I can not install it with the seatbelt FF in the captains chairs because they are way too contoured. This would not be an issue in your 2004 as the captains chairs are no where near contoured like mine
I get an acceptable FF install in the 3rd row 50/50 split bench but it is definitely more difficult.
I have been told that Dodge is not allowed to add top tether anchors to their vans after 2002 model years. However, I had two outboard anchors added to my 3rd row when I bought the van... I don't really think they know what they are talking about or what the rules are.
Dodge originally stated that the reason they could not add tether anchors to my 3rd row was because it was a crumple zone and carseats could not go back there. Then when I questioned whether it was safe for an adult to sit back there, they changed their mind and stated that I was not allowed to put three seats across the back row because I could not hook two carseats up to the same LATCH anchor... I pointed out that there were no LATCH anchors in my 3rd row and the seatbelts that were installed back there already would work just fine for installing carseats.
They ended up calling their corporate headquarters and I had the tether anchors installed the next day.
I would get Radians for your son and daughter in the captains chairs. When installed with LATCH you can flip the captains chair forward to access the third row with the Radian (or any FF LATCH carseat) still installed. They will definitely last the longest. My 5 year old has a 16 inch torso and still has 1 - 1.5 inches left to go in his Radian. He has outgrown all other harnessed seats on the market... he is right even with the MA's top slots in some vehicles but is over them when the MA is installed in my van.