I don't expect land plots to absorb 3, 4 or 5 cards each; any time soon.
I think we'll need to harvest resources for quite a while before we can upgrade the building and open up those multiple slots.
We might've sold out of the edition after Rebellion, before anyone assigns a fourth card to any plot.
Yeah, it is possible for sure. Is there any info on what it takes to upgrade buildings?
Nothing remotely solid.