I emailed Costco a few days ago to get clarification since their webpage is poorly written and doesn't state that you get only 4 years total anywhere if you bought the TV elsewhere. There's bold blocks and verbage stating 5 years total in several places. Here's their response:
If you purchased the TV through Costco.com or our Costco warehouse we match the manufacturer warranty a additional year for our TV's and with the square trade it adds a additional 3 years. Since you purchased your TV through a different company the square trade warranty only adds a additional 3 years to the manufacturers warranty. If you have any questions or concerns please do not hesitate to contact me directly.
Thank you for your patience and have a wonderful day.
Ecommerce Resolution Specialist