The ease of buckling a booster really depends more on the seat belt than the booster. I have older kiddos so we've been through quite a lot of this. In our VW, the boys could buckle themselves pretty early because the buckle stalk is stiff - they can push the buckle into it with one hand and the buckle doesn't fall over.
In our Astro, though, they had a much harder time. The Astro has locking latchplates (the "male" end) so they don't slide up and down the seat belt as easily, and the buckles are on short webbing, so not only are they hard to reach from up on a booster, you really need two hands to do it - one to hold the buckle and one to hold the latchplate.
And in our Mazda5, we have two rows of seats, and the 2nd row has stiff buckle stalks and the 3rd row has the floppy kind.

So even my 12yo has a more difficult time buckling himself in his booster in the 3rd row!
As others have already pointed out, the ideal solution would be to get a harnessed seat appropriate for your 4 year old's size. Most kid have the motor skills to properly buckle their own harness long before they can properly buckle a seat belt while in a booster.
If you're really opposed to that, you could try out some different boosters to see if a different style makes it easier. What booster is she currently in?
Also, if you do decide to keep her in a booster and you find one that she can buckle herself, keep in mind that you still need to check and make sure she's buckled properly. For example, a common problem with the Graco TurboBooster is that the seat belt doesn't get routed under the armrests. This is a very dangerous situation, and many children forget or don't notice that it's wrong. It's also hard for many younger children to pull the seat belt tight after they've buckled themselves, particularly if the seat belt has locking latchplates, which is obviously another serious risk - a loose seat belt may not do its job at all!
Another option might be to re-arrange the kids - can any of your younger children buckle their harnesses? You might be able to move one of them to the back and move your 4 year old up so she's easier to reach and help.
Oh, and what kind of minivan do you have? Does it have captain's chairs? I know a lot of people remove one of the captain's chairs and put two kids in the back; that way they can easily get in to buckle the kids in the 3rd row.