Wow that's pretty heavy, makes a 2014 GT500 look like a lightweight! Lots of cars these days weighing over 4,000 so no big surprise but it would have been nice to see it come under 4,000 lbs. I don't think there's going to be a lot of weight savings opportunities as the factory has already picked the low hanging fruit.
https://www.motorauthority.com/news/1124390_2020-ford-mustang-shelby-gt500-specs-deep-dive
https://www.motorauthority.com/news/1124390_2020-ford-mustang-shelby-gt500-specs-deep-dive