Hi weiji there will still be one more beta release before the final version is out. I don't have an exact date yet. It shouldn't be much longer but that's what I thought 7 months ago as well.
But yes if you purchase v3 now you will be able to get a free upgrade to v4 and you can also use the current v4 beta release at http://moi3d.com/beta.htm .
It's generally more of a focus to try and make v4 a high quality stable release than to rush it out. It takes a lot of time to make that happen.