starting a development with CI alpha.

I would start a new projet with CI4 :-) It would be easier than looking at a conversion part-way through a project.
CI3 will be around for some time to come, but I expect CI4 to get more attention over the next few years, and it will be around for some time after that.

Disclaimer: I have a slight bias

ps - the next release should be the first beta.

I would agree, I have a completed project already on ci4. I monitor changes to the framework itself. And by the way I did a little restructuring to suit my needs.