Ecosystem libraries would need to switch from a single peer dependency on Zod to two optional peer dependencies. Despite "optional peer dependencies" technicall being a thing, its functionally impossible for a library to determine which of the two packages your user is actually bringing to the table.

Let's say a library is trying to implement an `acceptSchema` function that can accepts `Zod3Type | Zod4Type`. For starters: those two interfaces won't both be available if Zod 3 and Zod 4 are in separate packages. So that's already a non-starter. And differentiating them at runtime requires knowing which package is installed, which is impossible in the general case (mostly because frontend bundlers generally have no affordance for optional peer dependencies).

I describe this in more detail here: https://x.com/colinhacks/status/1922101292849410256