Nuxt 4 production optimization: hydration, performance, testing with Vitest, deployment to Cloudflare/Vercel/Netlify, and v4 migration. Use when: debugging hydration mismatches, optimizing performance and Core Web Vitals, writing tests with Vitest, deploying to Cloudflare Pages/Workers/Vercel/Netlify, or migrating from Nuxt 3 to Nuxt 4. Keywords: hydration, hydration mismatch, ClientOnly, SSR, performance, lazy loading, lazy hydration, Vitest, testing, deployment, Cloudflare Pages, Cloudflare Workers, Vercel, Netlify, NuxtHub, migration, Nuxt 3 to Nuxt 4