couple things here: 1) 4runner v. mini van and 2) trade in for a new vehicle.
1) New sienna is very nice. There is no argument that the minivan is the best way to haul around lots of people, most of the time. You didn't mention how many kids and what age. The younger and closer in age the more reason to check out the van. Personally I have my Runner and a 08 honda oddy. We have a 4 year old girl and 2 year old twin boys. We have 1 seat in the middle row, and the full 3rd row in place. This gives a big area to walk into the van, close the door and get organized (diapers, food, get out of the elements, we even carry a potty along just in case for the older one, etc). Also our layout separates the kids so they cannot reach each other. Then come the kids' friends, you will have lots of quality seating in the van, no comparo to the suv. We frequently pull over and walk around inside the van...this is a great feature that no suv can match. Note, that van is BIG! make no mistake, make sure your wife will want to drive that large of a vehicle, or at least get used to driving it.
2) trade in. I assume you have had the suv for a while. I personally would not want to "loose" the depreciation you have already had on it. I would also not want to pay top dollar right now for a new van, and then again have the depreciation start on that vehicle.
The Runner is a unique vehicle that is great in many ways. If you tow, have lots of inclement weather, camp or have a remote cottage and 3 or less kids, you could stay with the runner for a while longer. You can then get the sienna used if you still want that. However, 3 or more kids, depending on the driving and what else you do (ferry around family, or kids' friends) the van might be a better choice.
Why does the wife want to trade? what are her reasons?
J