CodeIgniter 3 Development after v4 release?

I can't find the original roadmap thread that mentioned this, but yes - it will have active support for at least a year. And "active support" means not just security fixes, but I don't know what you mean by "active development".

I mean ... We'd have to discontinue it eventually - don't think that you'd be running it forever - but version 4 is a rewrite, so we know that adoption would take more time.