My understanding is that you can only buy the ESA once; regardless if it's the 2yr or 4yr.
The ESA is prorated refundable, *HOWEVER*, after prorated, any repairs you've had done using the ESA are subtracted from that.
In other words, you buy the 4yr ESA on 07/01/18 for $4,500. On 08/11/18 you have the MCU replaced under the ESA ($2700). On 09/23/18 the AC Compressor goes out, taking the DCDC Converter with it ($3800). On 11/01/18 you request a refund. The prorated refund is $4125. Subtract repair costs under ESA (4125 - 2700 - 3800) and you end-up at -$2375. No refund for you!