Hi Igor,
We have used 3.x, 4.5 pre alpha, 4.5 alpha, 4.5 beta and 4.5 stable on our production sites. A good number of them are in 4.5 beta. I think 4.5 has proven itself to be a production ready code. Also using 3.x and going to 4.x you will have to unlearn some stuff. I think its best for you start in 4.x since you have the benefit of having a fresh perspective.
