I think I see what Apple are planning here. Its clear that they want 4 tiers of phone, and I think the 18 series will be as follows:
Iphone NEO (replacing the 17e)
Iphone (a downgraded 17)
Iphone Pro / Pro Max 2027 (will basically be the current iPhone 17 as there was little difference in the 17 and 17 pro)
Iphone Ultra (The foldable iPhone)
4 clear tiers with a significant step up to the next model, and not the blurred line between the 17 and 17 pro lineup
Your four-tier speculation is plausible to me only because Apple introduced the iPhone Air as a fourth option. That broke the good/better/best tiering of the iPhone. Air was positioned between the base model (better) and the Pro (best), with good-level battery life and camera, best-level screen size and chip, and thinness as a new best-level feature. By failing to integrate thinness coherently into the purchase decision process, it made the iPhone Air look like a step up in price with a step down in function. The result was confusion and poor adoption.
Someone was too enamored of thinness and the Air moniker used for MacBooks and iPads. They were looking for reuse of the name as a simplistic branding idea instead of thinking about the logic of the product line. Apple should have anticipated bad sales. Marketing completely dropped the ball by failing to fully analyze the behavioral-marketing consequences.
Apple should restore the good/better/best product ladder. Figure out what to do with Air. Jettison the product or figure out how to place the feature as either part of the better or best tier. It should not further confuse things by introducing a fifth product (the foldable) above the best tier at twice the price.
A foldable phone sits between a phone and a tablet. It invites the consumer to ask a different question. Instead of "what phone do I want?" the consumer has to also consider "do I want or need a portable computing device?". If the answer to the last question is yes, then they have to ask "do I need a tablet or a laptop?" That question is stimulated not only by the overlap in function, but also by price. For less than the rumored base price of a foldable, I can buy an iPhone Pro and an iPad, or an iPhone Pro and a MacBook Neo, or an iPhone and a MacBook Air.
Apple shouldn't position a foldable phone as an iPhone Ultra. It should do what it has done well in the past. Create a new category that directly invites the question: "Why should I buy a separate laptop, tablet, and smartphone when I can get the function of all three for less money and have it fit in my pocket?" Apple should make sure the device has enough memory to support an external monitor, keyboard, and mouse. It should be able to run the Apple Creative Studio apps to give it full workflow parity with existing iPads and Macs. Basically, Apple has to do an Apple-class job on the device and not half-ass it because it thinks of the product as a higher-end iPhone. That would justify positioning as a low-end enterprise-class mobile computing device, not merely an ultra-premium iPhone.
Of course, it would require Apple to market it differently with a new name: Apple Momentum. Drop the "i". Drop the connection with only one function of the device. You get true momentum by having the center of your communication, social media, video conferencing, camera, navigation, and enterprise-class mobile computing in your pocket at all times.