Are you sure?
I don't see a way to get it through the rear door, there is no dimension even close to 4'!
Likewise diagonal seems 45" max
On the diagonal (front to back as well as side to side), it can barely fit. To be safe, I prefer putting it vertically behind the back seats better, because I don't want to mess up my upholstery.
Don't EVER try to put a bale of alfalfa in the back of a Model S. You can jam it in there, but you'll have to take it out piece by piece. It's a nightmare, I speak from personal experience. Took 2 months to remove all the alfalfa bits.