Car seat

This came up in another group I belong to & im just trying to see what others have done. I'm wondering how you have it set up. Our four year old has been behind the passenger for convinence to us (the driver) for easy view & accessibility. We're wondering weather now we should move her behind the driver & have the baby on the passenger side? I'm curious how it's worked for others & what your reasoning is for your placement of the car seats. I want what's safest, but also what's most practical.