We wrote a practical Developer Guide for Gemini 4 Carbon covering API changes, long decode up to 1M tokens, and multi-agent patterns: https://aistudio.google.com/learn/gemini-4-carbon-developer-guide
Upgrading from Argon means just changing the id to gemini-4-carbon. I use medium thinking_level for almost everything. For bigger multi-file tasks, the new /swarm preview in @Antigravity creates a multi-agent team that can communicate via a message board. You can put Carbon on architecture and code review while 3.8 Flash works on parallel edits.