V3x or V4

Hi, So I've made a decision and CodeIgniter is my weapon of choice (not Symfony or Laravel).

But now the big question, do I go with version CI 3.1x or CI4 ?

I would like to use the MYSQL PDO driver, but notice CI4 does not support this, I believe V3.1x does ?

Also, all of the documentation references "CodeIgniter 4 is the upcoming version of the framework". Does this mean it is not quite stable enough or something else?

Thank you for your input.