Apple added references to identifiers that include iPhone15,4, iPhone15,5, iPhone16,1, and iPhone16,2 in the first beta of tvOS 17 and they stuck around for several betas before Apple noticed and pulled them out in beta 5. The numbering fits with rumors that Apple will use the 3nm A17 chip for the iPhone 15 Pro and Pro Max, while the iPhone 15 and 15 Plus get the same A16 chip used in the iPhone 14 Pro and Pro Max.
15,4 and 15,5 are in the same sequence as the iPhone 14 Pro and Pro Max, which are iPhone15,2 and iPhone15,3, respectively. Apple uses the “15” to denote devices with the A16 chip. The iPhone 14 and iPhone 14 Plus, for example, are iPhone14,7 and iPhone14,8 because they’re using the A15.
So because the iPhone 15 and 15 Plus will get the A16 chip already used in the 14 Pro and Pro Max, the 15,4 and 15,5 likely reference these two devices. 16,1 and 16,2 will be the first iPhones with a new chip, aka the rumored A17, so these model numbers likely refer to the iPhone 15 Pro and iPhone 15 Pro Max.