I think it would be better (and probably cheaper) to get a 2018+ Pacifica with the 8.4 screen if that is what you want instead of changing the screen hardware. 2018+ Pacificas come with screens with Apple Car Play/Android Auto. (The 2017 Pacificas screens don't have Apple CarPlay/Android Auto.) Some vans have a navigation feature, but it isn't the same as phone navigation plugged into the screen using Apple CarPlay/Android Auto. You will have to pay ($100-150) UConnect for map updates when you want them with the van navigation feature that doesn't use Apple CarPlay/Android Auto.
If you are buying a used van, it will be hard to tell what the difference in price would be. When I bought my 2017 van (new in 2016) there was about a $595 price difference for getting the 8.4 nav optional screen instead of the 4-5 inch screen. The larger screen was worth it to me.
It is possible that the different screen size may have hidden features associated with it such as navigation and better speakers. You can check the VINs to see what the difference is for the used vans you are comparing. This link will get you a window sticker if you open it in a new tab with the VIN at the end.
There is another VIN look up tool for the build sheet. Add the VIN to the end.
If you are really interested in a 2017 and changing the screen, here is thread with about 150 posts that you can read .
Not sure what you're trying to do here. Are you trying to enable Carplay or Android Auto on it? If not, what is the difference between 3rd and 4th Gen ? I am also interested to see if we can enable Carplay on this. That was my goal, but the more I talked to folks it's not possible on the 2017...