If all four tires have been rotated according to the maintenance procedure, it is a little odd that only the rear tires need to be replaced. There is no safety issue as long as both tires on an axle are the same. That is, the left front and the right front must be the same, and the left rear and the right rear must be the same, but the tires on the two different axles can be different, within reason, of course.
I replaced mine with Goodyear Comfortreds and have been really pleased with all aspects of them. They are the most comfortable tires Goodyear makes, and they are rated for a life of 60,000 miles, which might see you to the end of ownership for your car.
Recommendation: I would ask the dealer to estimate the mileage left in the front tires so you can decide if you want to replace all four at the same time. I live in San Jose, so we might have bought our cars at the same dealership. I bought mine from Capitol Toyota. When I went there to ask for a price quotation on the Goodyear Comfortreds, their Parts Manager had no idea which ones I was talking about, so I went to Skip's Tires on Blossom Hill across from and just west of Oakridge Mall. Including filling the tires with Nitrogen, the whole bill was just over $500.
Jim