It is going to be tought to add ABS system into a car that was made for non ABS. Since your a i4, you have a 5sfe that came with front disc's and rear drums. The i4 Camrys that came equiped with ABS had rear disc's as well. For the price of a rear drum to disc conversion and then adding aftermarket ABS (since toyota one is not an option IMO) your pretty much spending close to a grand (since you asked toyota dealership). For that price, i'd just buy good tires and ensure your current braking system is not worn and is great working condition (pads have life and rotors resurfaced, and e-brake reconditioned and/or re-adjusted).