The part number is the same as I bought for my 2015....00544-MF240-575.
In Dec 2019 I bought it from my local dealer and paid $156.80 with tax and got back $16 for the core charge. I installed it myself for a total of $140.80.

 

The battery in my car is identified as 80D26R-MF. I can't find batteries with this code at Interstate or Auto Zone or O'reilly's.

 

How about this:

 

I guess that is my only option, however, it differs from what came with the car in that it is not maintenance-free, and it's 2 1/2" taller.

 

Yours is only around 6" tall? I measured mine and its roughly 8 so maybe you have enough head room to get a taller one.

 

Yep that's the one.

 

My 2015 battery finally started to fail and could not get the car to start this week. Costco has the Interstate as others have suggested. Here in NorCal, the cost is about $120. The battery has 700 CCA but because of the size, I would have to discard the battery housing in the engine compartment due to battery size -- did not want to do that. I chose the Lexus 84-month battery by Interstate, the same part number stated by others. The battery at my dealership was about $136.00 before core and taxes. The only drawback is that the Lexus battery has only 575 CCA. Much less the Costco's but with a longer pro-rated warranty. Since the original battery lasted 8 years with 580 CCA, crossing my fingers on the replacement. I have replaced several batteries in the past but this one took me an hour just trying to get the harness and fitting it back in with the new battery. It was difficult but I was able to get the casing onto the battery before putting them into the car and snapping the 3 harnesses. Everyone mentioned 2 harnesses but on the GS there are 3 harness clips. Is there much difference between the 575 vs 700 CCA? Is it how long I can run the radio without turning on the car? Can someone educate me on this? I want the Costco battery if it was not for the casing and battery holder. No issues electronically after the replacement -- hope it stays that way.

 

You'll never EVER experience the difference! Cold-cranking amperage is only an important factor in extremely cold climates - it measures the amperage available from a fully charged battery over 30 seconds of time at 0°F without dropping below 7.2 volts. A comparison of two batteries with different cold-cranking amperages at any temperature above about 50° becomes essentially the same - as neither of the two batteries will drop remotely close to the 7.2 volts CCA measurement standard. It would be like comparing a glass of 58°F water to a glass of 60°F water to see which one of the two glasses of water freezes the fastest at a temperature of 31°F.

As for how long you can run your radio without turning on the car. Your 4GS will conveniently turn Off your radio after about 20 minutes to protect the battery. Same with the door courtesy lights if a passenger door or the trunk is left open.