Proposal: Update Zod to 4.0

[Nlea]

• Upgrade zod dependency to ^4.0.0.
• Adjust any code that breaks due to API changes.

[Nlea]

#108 updates the non-openapi templates. The openapi templates are still on Zod 3 for now

[Nlea]

hono-openapi@1.1.0 cannot be used in the openapi templates with Zod v4, despite zod-openapi@5.x supporting it. The issue is caused by transitive peer dependency constraints.

Dependency Chain

The problem occurs in the following dependency chain:

hono-openapi@1.1.0
└─ peerDependency: @standard-community/standard-openapi@^0.2.8
└─ peerOptional: zod-openapi@^4
└─ peerDependency: zod@^3.21.4

The Conflict

zod-openapi@5.x supports Zod v4 ✅
@standard-community/standard-openapi@0.2.8 only accepts zod-openapi@^4 (not v5) ❌
zod-openapi@4.x only supports zod@^3.x ❌
This creates an incompatibility: even though a Zod v4-compatible version of zod-openapi exists, it cannot be used because @standard-community/standard-openapi@0.2.8 restricts it to v4.

Attempted Workarounds
Using npm overrides

json
{
  "dependencies": {
    "zod": "^4.1.11",
    "zod-openapi": "^5.4.2"
  },
  "overrides": {
    "zod-openapi": "^5.4.2"
  }
}

This allows installation but causes runtime errors:

✘ [ERROR] Could not resolve "zod-openapi/extend"
  The path "./extend" is not exported by package "zod-openapi"

✘ [ERROR] Could not resolve "hono-openapi/zod"
  The path "./zod" is not exported by package "hono-openapi"
The APIs between zod-openapi@4 and zod-openapi@5 are incompatible.

Current Workaround

Use Zod v3 until hono-openapi updates to a version of standard-community that supports zod4.