To start with, the Ag tires are quite probably an entirely different size than the turfs or R4s, and are meant to fit a narrower two piece rim, not the welded type the R4s go on.
I seriously doubt you're going to get 4 new tires and rims for $400, to me that is the price difference, expressed either as a credit or an additional cost when you specify a different tire/rim combination on a new tractor order.
Mine came with AG tires as standard equipment, the dealer told me they typically order in only R4 equipped tractors, that is their best seller. Final price was the same, they were prepared to switch to my tire of choice at no additional cost (or credit) to me.
I've never seen such a simple question draw so many answers that are pure speculation !!
Chilly